xref: /dragonfly/sbin/hammer/hammer.8 (revision fcf53d9b)

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.Dd April 19, 2011
.Dt HAMMER 8
.Os
.Sh NAME
.Nm hammer
.Nd HAMMER file system utility
.Sh SYNOPSIS
.Nm
.Fl h
.Nm
.Op Fl 2BFqrvXy
.Op Fl b Ar bandwidth
.Op Fl C Ar cachesize Ns Op Ns Cm \&: Ns Ar readahead
.Op Fl c Ar cyclefile
.Op Fl f Ar blkdevs
.\" .Op Fl s Ar linkpath
.Op Fl i Ar delay
.Op Fl p Ar ssh-port
.Op Fl S Ar splitsize
.Op Fl t Ar seconds
.Ar command
.Op Ar argument ...
.Sh DESCRIPTION
This manual page documents the
.Nm
utility which provides miscellaneous functions related to managing a
.Nm HAMMER
file system.
For a general introduction to the
.Nm HAMMER
file system, its features, and
examples on how to set up and maintain one, see
.Xr HAMMER 5 .
.Pp
The options are as follows:
.Bl -tag -width indent
.It Fl 2
Tell the mirror commands to use a 2-way protocol, which allows
automatic negotiation of transaction id ranges.
This option is automatically enabled by the
.Cm mirror-copy
command.
.It Fl B
Bulk transfer.
.Cm Mirror-stream
will not attempt to break-up large initial bulk transfers into smaller
pieces.
This can save time but if the link is lost in the middle of the
initial bulk transfer you will have to start over from scratch.
For more information see the
.Fl S
option.
.It Fl b Ar bandwidth
Specify a bandwidth limit in bytes per second for mirroring streams.
This option is typically used to prevent batch mirroring operations from
loading down the machine.
The bandwidth may be suffixed with
.Cm k , m ,
or
.Cm g
to specify values in kilobytes, megabytes, and gigabytes per second.
If no suffix is specified, bytes per second is assumed.
.Pp
Unfortunately this is only applicable to the pre-compression bandwidth
when compression is used, so a better solution would probably be to
use a
.Xr ipfw 8
pipe or a
.Xr pf 4
queue.
.It Fl C Ar cachesize Ns Op Ns Cm \&: Ns Ar readahead
Set the memory cache size for any raw
.Tn I/O .
The default is 16MB.
A suffix of
.Cm k
for kilobytes and
.Cm m
for megabytes is allowed,
else the cache size is specified in bytes.
.Pp
The read-behind/read-ahead defaults to 4
.Nm HAMMER
blocks.
.Pp
This option is typically only used with diagnostic commands
as kernel-supported commands will use the kernel's buffer cache.
.It Fl c Ar cyclefile
When pruning, rebalancing or reblocking you can tell the utility
to start at the object id stored in the specified file.
If the file does not exist
.Nm
will start at the beginning.
If
.Nm
is told to run for a specific period of time
.Pq Fl t
and is unable to complete the operation it will write out
the current object id so the next run can pick up where it left off.
If
.Nm
runs to completion it will delete
.Ar cyclefile .
.It Fl F
Force operation.
E.g.\&
.Cm cleanup
will not check that time period has elapsed if this option is given.
.It Fl f Ar blkdevs
Specify the volumes making up a
.Nm HAMMER
file system.
.Ar Blkdevs
is a colon-separated list of devices, each specifying a
.Nm HAMMER
volume.
.It Fl h
Show usage.
.It Fl i Ar delay
Specify delay in seconds for
.Cm mirror-read-stream .
When maintaining a streaming mirroring this option specifies the
minimum delay after a batch ends before the next batch is allowed
to start.
The default is five seconds.
.It Fl p Ar ssh-port
Pass the
.Fl p Ar ssh-port
option to
.Xr ssh 1
when using a remote
specification for the source and/or destination.
.It Fl q
Decrease verboseness.
May be specified multiple times.
.It Fl r
Specify recursion for those commands which support it.
.It Fl S Ar splitsize
Specify the bulk splitup size in bytes for mirroring streams.
When a
.Cm mirror-stream
is first started
.Nm
will do an initial run-through of the data to calculate good
transaction ids to cut up the bulk transfers, creating
restart points in case the stream is interrupted.
If we don't do this and the stream is interrupted it might
have to start all over again.
The default is a
.Ar splitsize
of 4GB.
.Pp
At the moment the run-through is disk-bandwidth-heavy but some
future version will limit the run-through to just the B-Tree
records and not the record data.
.Pp
The splitsize may be suffixed with
.Cm k , m ,
or
.Cm g
to specify values in kilobytes, megabytes, or gigabytes.
If no suffix is specified, bytes is assumed.
.Pp
When mirroring very large filesystems the minimum recommended
split size is 4GB.
A small split size may wind up generating a great deal of overhead
but very little actual incremental data and is not recommended.
.It Fl t Ar seconds
Specify timeout in seconds.
When pruning, rebalancing, reblocking or mirror-reading
you can tell the utility to stop after a certain period of time.
A value of 0 means unlimited.
This option is used along with the
.Fl c Ar cyclefile
option to prune, rebalance or reblock incrementally.
.It Fl v
Increase verboseness.
May be specified multiple times.
.It Fl X
Enable compression for any remote ssh specifications.
This option is typically used with the mirroring directives.
.It Fl y
Force
.Dq yes
for interactive questions.
.El
.Pp
The commands are as follows:
.Bl -tag -width indent
.\" ==== synctid ====
.It Cm synctid Ar filesystem Op Cm quick
Generate a guaranteed, formal 64-bit transaction id representing the
current state of the specified
.Nm HAMMER
file system.
The file system will be synced to the media.
.Pp
If the
.Cm quick
keyword is specified the file system will be soft-synced, meaning that a
crash might still undo the state of the file system as of the transaction
id returned but any new modifications will occur after the returned
transaction id as expected.
.Pp
This operation does not create a snapshot.
It is meant to be used
to track temporary fine-grained changes to a subset of files and
will only remain valid for
.Ql @@
access purposes for the
.Cm prune-min
period configured for the PFS.
If you desire a real snapshot then the
.Cm snapq
directive may be what you are looking for.
.\" ==== bstats ====
.It Cm bstats Op Ar interval
Output
.Nm HAMMER
B-Tree statistics until interrupted.
Pause
.Ar interval
seconds between each display.
The default interval is one second.
.\" ==== iostats ====
.It Cm iostats Op Ar interval
Output
.Nm HAMMER
.Tn I/O
statistics until interrupted.
Pause
.Ar interval
seconds between each display.
The default interval is one second.
.\" ==== history ====
.It Cm history Ns Oo Cm @ Ns Ar offset Ns Oo Cm \&, Ns Ar length Oc Oc Ar path ...
Show the modification history for inode and data of
.Nm HAMMER
files.
If
.Ar offset
is given history is shown for data block at given offset,
otherwise history is shown for inode.
If
.Fl v
is specified
.Ar length
data bytes at given offset are dumped for each version,
default is 32.
.Pp
For each
.Ar path
this directive shows object id and sync status,
and for each object version it shows transaction id and time stamp.
Files has to exist for this directive to be applicable,
to track inodes which has been deleted or renamed see
.Xr undo 1 .
.\" ==== blockmap ====
.It Cm blockmap
Dump the blockmap for the file system.
The
.Nm HAMMER
blockmap is two-layer
blockmap representing the maximum possible file system size of 1 Exabyte.
Needless to say the second layer is only present for blocks which exist.
.Nm HAMMER Ns 's
blockmap represents 8-Megabyte blocks, called big-blocks.
Each big-block has an append
point, a free byte count, and a typed zone id which allows content to be
reverse engineered to some degree.
.Pp
In
.Nm HAMMER
allocations are essentially appended to a selected big-block using
the append offset and deducted from the free byte count.
When space is freed the free byte count is adjusted but
.Nm HAMMER
does not track holes in big-blocks for reallocation.
A big-block must be completely freed, either
through normal file system operations or through reblocking, before
it can be reused.
.Pp
Data blocks can be shared by deducting the space used from the free byte
count for each shared references.
This means the free byte count can legally go negative.
.Pp
This command needs the
.Fl f Ar blkdevs
option.
.\" ==== checkmap ====
.It Cm checkmap
Check the blockmap allocation count.
.Nm
will scan the B-Tree, collect allocation information, and
construct a blockmap in-memory.
It will then check that blockmap against the on-disk blockmap.
.Pp
This command needs the
.Fl f Ar blkdevs
option.
.\" ==== show ====
.It Cm show Op Ar localization Ns Op Cm \&: Ns Ar object_id
Dump the B-Tree.
By default this command will validate all B-Tree
linkages and CRCs, including data CRCs, and will report the most verbose
information it can dig up.
Any errors will show up with a
.Ql B
in column 1 along with various
other error flags.
.Pp
If you specify
.Ar localization
or
.Ar localization Ns Cm \&: Ns Ar object_id
the dump will
search for the key printing nodes as it recurses down, and then
will iterate forwards.
These fields are specified in HEX.
Note that the pfsid is the top 16 bits of the 32-bit localization
field so PFS #1 would be 00010000.
.Pp
If you use
.Fl q
the command will report less information about the inode contents.
.Pp
If you use
.Fl qq
the command will not report the content of the inode or other typed
data at all.
.Pp
If you use
.Fl qqq
the command will not report volume header information, big-block fill
ratios, mirror transaction ids, or report or check data CRCs.
B-Tree CRCs and linkages are still checked.
.Pp
This command needs the
.Fl f Ar blkdevs
option.
.\" ==== show-undo ====
.It Cm show-undo
.Nm ( HAMMER
VERSION 4+)
Dump the UNDO/REDO map.
.Pp
This command needs the
.Fl f Ar blkdevs
option.
.\" .It Ar blockmap
.\" Dump the B-Tree, record, large-data, and small-data blockmaps, showing
.\" physical block assignments and free space percentages.
.\" ==== recover ====
.It Cm recover Ar targetdir
Recover data from a corrupted
.Nm HAMMER
filesystem.
This is a low level command which operates on the filesystem image and
attempts to locate and recover files from a corrupted filesystem.
The entire image is scanned linearly looking for B-Tree nodes.
Any node
found which passes its CRC test is scanned for file, inode, and directory
fragments and the target directory is populated with the resulting data.
files and directories in the target directory are initially named after
the object id and are renamed as fragmentary information is processed.
.Pp
This command keeps track of filename/object_id translations and may eat a
considerably amount of memory while operating.
.Pp
This command is literally the last line of defense when it comes to
recovering data from a dead filesystem.
.Pp
This command needs the
.Fl f Ar blkdevs
option.
.\" ==== namekey1 ====
.It Cm namekey1 Ar filename
Generate a
.Nm HAMMER
64-bit directory hash for the specified file name, using
the original directory hash algorithm in version 1 of the file system.
The low 32 bits are used as an iterator for hash collisions and will be
output as 0.
.\" ==== namekey2 ====
.It Cm namekey2 Ar filename
Generate a
.Nm HAMMER
64-bit directory hash for the specified file name, using
the new directory hash algorithm in version 2 of the file system.
The low 32 bits are still used as an iterator but will start out containing
part of the hash key.
.\" ==== namekey32 ====
.It Cm namekey32 Ar filename
Generate the top 32 bits of a
.Nm HAMMER
64 bit directory hash for the specified file name.
.\" ==== info ====
.It Cm info
Show extended information about
.Nm HAMMER
file systems.
The information is divided into sections:
.Bl -tag -width indent
.It Volume identification
General information, like the label of the
.Nm HAMMER
filesystem, the number of volumes it contains, the FSID, and the
.Nm HAMMER
version being used.
.It Big block information
Big block statistics, such as total, used, reserved and free big blocks.
.It Space information
Information about space used on the filesystem.
Currently total size, used, reserved and free space are displayed.
.It PFS information
Basic information about the PFSs currently present on a
.Nm HAMMER
filesystem.
.Pp
.Dq PFS ID
is the ID of the PFS, with 0 being the root PFS.
.Dq Snaps
is the current snapshot count on the PFS.
.Dq Mounted on
displays the mount point of the PFS is currently mounted on (if any).
.El
.\" ==== cleanup ====
.It Cm cleanup Op Ar filesystem ...
This is a meta-command which executes snapshot, prune, rebalance, dedup
and reblock commands on the specified
.Nm HAMMER
file systems.
If no
.Ar filesystem
is specified this command will clean-up all
.Nm HAMMER
file systems in use, including PFS's.
To do this it will scan all
.Nm HAMMER
and
.Nm null
mounts, extract PFS id's, and clean-up each PFS found.
.Pp
This command will access a snapshots
directory and a configuration file for each
.Ar filesystem ,
creating them if necessary.
.Bl -tag -width indent
.It Nm HAMMER No version 2-
The configuration file is
.Pa config
in the snapshots directory which defaults to
.Pa <pfs>/snapshots .
.It Nm HAMMER No version 3+
The configuration file is saved in file system meta-data, see
.Nm
.Cm config .
The snapshots directory defaults to
.Pa /var/hammer/<pfs>
.Pa ( /var/hammer/root
for root mount).
.El
.Pp
The format of the configuration file is:
.Bd -literal -offset indent
snapshots  <period> <retention-time> [any]
prune      <period> <max-runtime>
rebalance  <period> <max-runtime>
dedup      <period> <max-runtime>
reblock    <period> <max-runtime>
recopy     <period> <max-runtime>
.Ed
.Pp
Defaults are:
.Bd -literal -offset indent
snapshots  1d 60d  # 0d 0d  for PFS /tmp, /var/tmp, /usr/obj
prune      1d 5m
rebalance  1d 5m
dedup      1d 5m
reblock    1d 5m
recopy     30d 10m
.Ed
.Pp
Time is given with a suffix of
.Cm d ,
.Cm h ,
.Cm m
or
.Cm s
meaning day, hour, minute and second.
.Pp
If the
.Cm snapshots
directive has a period of 0 and a retention time of 0
then snapshot generation is disabled, removal of old snapshots are
disabled, and prunes will use
.Cm prune-everything .
.Pp
If the
.Cm snapshots
directive has a period of 0 but a non-zero retention time
then this command will not create any new snapshots but will remove old
snapshots it finds based on the retention time.
This form should be
used on PFS masters where you are generating your own snapshot softlinks
manually and on PFS slaves when all you wish to do is prune away existing
snapshots inherited via the mirroring stream.
.Pp
By default only snapshots in the form
.Ql snap- Ns Ar yyyymmdd Ns Op - Ns Ar HHMM
are processed.
If the
.Cm any
directive is specified as a third argument on the
.Cm snapshots
config line then any softlink of the form
.Ql *- Ns Ar yyyymmdd Ns Op - Ns Ar HHMM
or
.Ql *. Ns Ar yyyymmdd Ns Op - Ns Ar HHMM
will be processed.
.Pp
A period of 0 for prune, rebalance, reblock or recopy disables the directive.
A max-runtime of 0 means unlimited.
.Pp
If period hasn't passed since the previous
.Cm cleanup
run nothing is done.
For example a day has passed when midnight is passed (localtime).
If the
.Fl F
flag is given the period is ignored.
By default,
.Dx
is set up to run
.Nm Cm cleanup
nightly via
.Xr periodic 8 .
.Pp
The default configuration file will create a daily snapshot, do a daily
pruning, rebalancing, deduping and reblocking run and a monthly recopy run.
Reblocking is defragmentation with a level of 95%,
and recopy is full defragmentation.
.Pp
By default prune, dedup and rebalance operations are time limited to 5 minutes,
and reblock operations to a bit over 5 minutes,
and recopy operations to a bit over 10 minutes.
Reblocking and recopy runs are each broken down into four separate functions:
btree, inodes, dirs and data.
Each function is time limited to the time given in the configuration file,
but the btree, inodes and dirs functions usually does not take very long time,
full defragmentation is always used for these three functions.
Also note that this directive will by default disable snapshots on
the following PFS's:
.Pa /tmp ,
.Pa /var/tmp
and
.Pa /usr/obj .
.Pp
The defaults may be adjusted by modifying the configuration file.
The pruning and reblocking commands automatically maintain a cyclefile
for incremental operation.
If you interrupt (^C) the program the cyclefile will be updated,
but a sub-command
may continue to run in the background for a few seconds until the
.Nm HAMMER
ioctl detects the interrupt.
The
.Cm snapshots
PFS option can be set to use another location for the snapshots directory.
.Pp
Work on this command is still in progress.
Expected additions:
An ability to remove snapshots dynamically as the
file system becomes full.
.\" ==== config ====
.It Cm config Op Ar filesystem Op Ar configfile
.Nm ( HAMMER
VERSION 3+)
Show or change configuration for
.Ar filesystem .
If zero or one arguments are specified this function dumps the current
configuration file to stdout.
Zero arguments specifies the PFS containing the current directory.
This configuration file is stored in file system meta-data.
If two arguments are specified this function installs a new config file.
.Pp
In
.Nm HAMMER
versions less than 3 the configuration file is by default stored in
.Pa <pfs>/snapshots/config ,
but in all later versions the configuration file is stored in file system
meta-data.
.\" ==== viconfig ====
.It Cm viconfig Op Ar filesystem
.Nm ( HAMMER
VERSION 3+)
Edit the configuration file and reinstall into file system meta-data when done.
Zero arguments specifies the PFS containing the current directory.
.\" ==== volume-add ====
.It Cm volume-add Ar device Ar filesystem
Add volume
.Ar device
to
.Ar filesystem .
This will format
.Ar device
and add all of its space to
.Ar filesystem .
A
.Nm HAMMER
file system can use up to 256 volumes.
.Pp
.Em NOTE!
All existing data contained on
.Ar device
will be destroyed by this operation!
If
.Ar device
contains a valid
.Nm HAMMER
file system, formatting will be denied.
You can overcome this sanity check by using
.Xr dd 1
to erase the beginning sectors of the device.
.Pp
Remember that you have to specify
.Ar device ,
together with any other device that make up the file system,
colon-separated to
.Pa /etc/fstab
and
.Xr mount_hammer 8 .
If
.Ar filesystem
is root file system, also remember to add
.Ar device
to
.Va vfs.root.mountfrom
in
.Pa /boot/loader.conf ,
see
.Xr loader 8 .
.\" ==== volume-del ====
.It Cm volume-del Ar device Ar filesystem
Remove volume
.Ar device
from
.Ar filesystem .
.Pp
Remember that you have to remove
.Ar device
from the colon-separated list in
.Pa /etc/fstab
and
.Xr mount_hammer 8 .
If
.Ar filesystem
is root file system, also remember to remove
.Ar device
from
.Va vfs.root.mountfrom
in
.Pa /boot/loader.conf ,
see
.Xr loader 8 .
.\" ==== volume-list ====
.It Cm volume-list Ar filesystem
List the volumes that make up
.Ar filesystem .
.\" ==== snapshot ====
.It Cm snapshot Oo Ar filesystem Oc Ar snapshot-dir
.It Cm snapshot Ar filesystem Ar snapshot-dir Op Ar note
Take a snapshot of the file system either explicitly given by
.Ar filesystem
or implicitly derived from the
.Ar snapshot-dir
argument and creates a symlink in the directory provided by
.Ar snapshot-dir
pointing to the snapshot.
If
.Ar snapshot-dir
is not a directory, it is assumed to be a format string passed to
.Xr strftime 3
with the current time as parameter.
If
.Ar snapshot-dir
refers to an existing directory, a default format string of
.Ql snap-%Y%m%d-%H%M
is assumed and used as name for the newly created symlink.
.Pp
Snapshot is a per PFS operation, so each PFS in a
.Nm HAMMER
file system have to be snapshot separately.
.Pp
Example, assuming that
.Pa /mysnapshots
is on file system
.Pa /
and that
.Pa /obj
and
.Pa /usr
are file systems on their own, the following invocations:
.Bd -literal -offset indent
hammer snapshot /mysnapshots

hammer snapshot /mysnapshots/%Y-%m-%d

hammer snapshot /obj /mysnapshots/obj-%Y-%m-%d

hammer snapshot /usr /my/snaps/usr "note"
.Ed
.Pp
Would create symlinks similar to:
.Bd -literal -offset indent
/mysnapshots/snap-20080627-1210 -> /@@0x10d2cd05b7270d16

/mysnapshots/2008-06-27 -> /@@0x10d2cd05b7270d16

/mysnapshots/obj-2008-06-27 -> /obj@@0x10d2cd05b7270d16

/my/snaps/usr/snap-20080627-1210 -> /usr@@0x10d2cd05b7270d16
.Ed
.Pp
When run on a
.Nm HAMMER
version 3+ file system the snapshot is also recorded in file system meta-data
along with the optional
.Ar note .
See the
.Cm snapls
directive.
.\" ==== snap* ====
.It Cm snap Ar path Op Ar note
.Nm ( HAMMER
VERSION 3+)
Create a snapshot for the PFS containing
.Ar path
and create a snapshot softlink.
If the path specified is a
directory a standard snapshot softlink will be created in the directory.
The snapshot softlink points to the base of the mounted PFS.
.It Cm snaplo Ar path Op Ar note
.Nm ( HAMMER
VERSION 3+)
Create a snapshot for the PFS containing
.Ar path
and create a snapshot softlink.
If the path specified is a
directory a standard snapshot softlink will be created in the directory.
The snapshot softlink points into the directory it is contained in.
.It Cm snapq Ar dir Op Ar note
.Nm ( HAMMER
VERSION 3+)
Create a snapshot for the PFS containing the specified directory but do
not create a softlink.
Instead output a path which can be used to access
the directory via the snapshot.
.Pp
An absolute or relative path may be specified.
The path will be used as-is as a prefix in the path output to stdout.
As with the other
snap and snapshot directives the snapshot transaction id will be registered
in the file system meta-data.
.It Cm snaprm Ar path Ar ...
.It Cm snaprm Ar transaction_id Ar ...
.It Cm snaprm Ar filesystem Ar transaction_id Ar ...
.Nm ( HAMMER
VERSION 3+)
Remove a snapshot given its softlink or transaction id.
If specifying a transaction id
the snapshot is removed from file system meta-data but you are responsible
for removing any related softlinks.
.Pp
If a softlink path is specified the filesystem and transaction id
is derived from the contents of the softlink.
If just a transaction id is specified it is assumed to be a snapshot in the
.Nm HAMMER
filesystem you are currently chdir'd into.
You can also specify the filesystem and transaction id explicitly.
.It Cm snapls Op Ar path ...
.Nm ( HAMMER
VERSION 3+)
Dump the snapshot meta-data for PFSs containing each
.Ar path
listing all available snapshots and their notes.
If no arguments are specified snapshots for the PFS containing the
current directory are listed.
This is the definitive list of snapshots for the file system.
.\" ==== prune ====
.It Cm prune Ar softlink-dir
Prune the file system based on previously created snapshot softlinks.
Pruning is the act of deleting file system history.
The
.Cm prune
command will delete file system history such that
the file system state is retained for the given snapshots,
and all history after the latest snapshot.
By setting the per PFS parameter
.Cm prune-min ,
history is guaranteed to be saved at least this time interval.
All other history is deleted.
.Pp
The target directory is expected to contain softlinks pointing to
snapshots of the file systems you wish to retain.
The directory is scanned non-recursively and the mount points and
transaction ids stored in the softlinks are extracted and sorted.
The file system is then explicitly pruned according to what is found.
Cleaning out portions of the file system is as simple as removing a
snapshot softlink and then running the
.Cm prune
command.
.Pp
As a safety measure pruning only occurs if one or more softlinks are found
containing the
.Ql @@
snapshot id extension.
Currently the scanned softlink directory must contain softlinks pointing
to a single
.Nm HAMMER
mount.
The softlinks may specify absolute or relative paths.
Softlinks must use 20-character
.Ql @@0x%016llx
transaction ids, as might be returned from
.Nm Cm synctid Ar filesystem .
.Pp
Pruning is a per PFS operation, so each PFS in a
.Nm HAMMER
file system have to be pruned separately.
.Pp
Note that pruning a file system may not immediately free-up space,
though typically some space will be freed if a large number of records are
pruned out.
The file system must be reblocked to completely recover all available space.
.Pp
Example, lets say your that you didn't set
.Cm prune-min ,
and snapshot directory contains the following links:
.Bd -literal -offset indent
lrwxr-xr-x  1 root  wheel  29 May 31 17:57 snap1 ->
/usr/obj/@@0x10d2cd05b7270d16

lrwxr-xr-x  1 root  wheel  29 May 31 17:58 snap2 ->
/usr/obj/@@0x10d2cd13f3fde98f

lrwxr-xr-x  1 root  wheel  29 May 31 17:59 snap3 ->
/usr/obj/@@0x10d2cd222adee364
.Ed
.Pp
If you were to run the
.Cm prune
command on this directory, then the
.Nm HAMMER
.Pa /usr/obj
mount will be pruned to retain the above three snapshots.
In addition, history for modifications made to the file system older than
the oldest snapshot will be destroyed and history for potentially fine-grained
modifications made to the file system more recently than the most recent
snapshot will be retained.
.Pp
If you then delete the
.Pa snap2
softlink and rerun the
.Cm prune
command,
history for modifications pertaining to that snapshot would be destroyed.
.Pp
In
.Nm HAMMER
file system versions 3+ this command also scans the snapshots stored
in the file system meta-data and includes them in the prune.
.\" ==== prune-everything ====
.It Cm prune-everything Ar filesystem
Remove all historical records from
.Ar filesystem .
Use this directive with caution on PFSs where you intend to use history.
.Pp
This command does not remove snapshot softlinks but will delete all
snapshots recorded in file system meta-data (for file system version 3+).
The user is responsible for deleting any softlinks.
.Pp
Pruning is a per PFS operation, so each PFS in a
.Nm HAMMER
file system have to be pruned separately.
.\" ==== rebalance ====
.It Cm rebalance Ar filesystem Op Ar saturation_percentage
Rebalance the B-Tree, nodes with small number of
elements will be combined and element counts will be smoothed out
between nodes.
.Pp
The saturation percentage is between 50% and 100%.
The default is 85% (the
.Sq %
suffix is not needed).
.Pp
Rebalancing is a per PFS operation, so each PFS in a
.Nm HAMMER
file system have to be rebalanced separately.
.\" ==== dedup ====
.It Cm dedup Ar filesystem
.Nm ( HAMMER
VERSION 5+)
Perform offline (post-process) deduplication.
Deduplication occurs at
the block level, currently only data blocks of the same size can be
deduped, metadata blocks can not.
The hash function used for comparing
data blocks is CRC-32 (CRCs are computed anyways as part of
.Nm HAMMER
data integrity features, so there's no additional overhead).
Since CRC is a weak hash function a byte-by-byte comparison is done
before actual deduping.
In case of a CRC collision (two data blocks have the same CRC
but different contents) the checksum is upgraded to SHA-256.
.Pp
Currently
.Nm HAMMER
reblocker may partially blow up (re-expand) dedup (reblocker's normal
operation is to reallocate every record, so it's possible for deduped
blocks to be re-expanded back).
.Pp
Deduplication is a per PFS operation, so each PFS in a
.Nm HAMMER
file system have to be deduped separately.
This also
means that if you have duplicated data in two different PFSs that data
won't be deduped, however the addition of such feature is planned.
.\" ==== dedup-simulate ====
.It Cm dedup-simulate Ar filesystem
Shows potential space savings (simulated dedup ratio) one can get after
running
.Cm dedup
command.
If the estimated dedup ratio is greater than 1.00 you will see
dedup space savings.
Remember that this is an estimated number, in
practice real dedup ratio will be slightly smaller because of
.Nm HAMMER
bigblock underflows, B-Tree locking issues and other factors.
.Pp
Note that deduplication currently works only on bulk data so if you
try to run
.Cm dedup-simulate
or
.Cm dedup
commands on a PFS that contains metadata only (directory entries,
softlinks) you will get a 0.00 dedup ratio.
.\" ==== reblock* ====
.It Cm reblock Ar filesystem Op Ar fill_percentage
.It Cm reblock-btree Ar filesystem Op Ar fill_percentage
.It Cm reblock-inodes Ar filesystem Op Ar fill_percentage
.It Cm reblock-dirs Ar filesystem Op Ar fill_percentage
.It Cm reblock-data Ar filesystem Op Ar fill_percentage
Attempt to defragment and free space for reuse by reblocking a live
.Nm HAMMER
file system.
Big-blocks cannot be reused by
.Nm HAMMER
until they are completely free.
This command also has the effect of reordering all elements, effectively
defragmenting the file system.
.Pp
The default fill percentage is 100% and will cause the file system to be
completely defragmented.
All specified element types will be reallocated and rewritten.
If you wish to quickly free up space instead try specifying
a smaller fill percentage, such as 90% or 80% (the
.Sq %
suffix is not needed).
.Pp
Since this command may rewrite the entire contents of the disk it is
best to do it incrementally from a
.Xr cron 8
job along with the
.Fl c Ar cyclefile
and
.Fl t Ar seconds
options to limit the run time.
The file system would thus be defragmented over long period of time.
.Pp
It is recommended that separate invocations be used for each data type.
B-Tree nodes, inodes, and directories are typically the most important
elements needing defragmentation.
Data can be defragmented over a longer period of time.
.Pp
Reblocking is a per PFS operation, so each PFS in a
.Nm HAMMER
file system have to be reblocked separately.
.\" ==== pfs-status ====
.It Cm pfs-status Ar dirpath ...
Retrieve the mirroring configuration parameters for the specified
.Nm HAMMER
file systems or pseudo-filesystems (PFS's).
.\" ==== pfs-master ====
.It Cm pfs-master Ar dirpath Op Ar options
Create a pseudo-filesystem (PFS) inside a
.Nm HAMMER
file system.
Up to 65536 PFSs can be created.
Each PFS uses an independent inode numbering space making it suitable
for replication.
.Pp
The
.Cm pfs-master
directive creates a PFS that you can read, write, and use as a mirroring
source.
.Pp
A PFS can only be truly destroyed with the
.Cm pfs-destroy
directive.
Removing the softlink will not destroy the underlying PFS.
.Pp
A PFS can only be created in the root PFS (PFS# 0),
not in a PFS created by
.Cm pfs-master
or
.Cm pfs-slave
(PFS# >0).
.Pp
It is recommended that
.Ar dirpath
is of the form
.Pa <fs>/pfs/<name>
(i.e.\& located in
.Pa pfs
directory at root of
.Nm HAMMER
file system).
.Pp
It is recommended to use a
.Nm null
mount to access a PFS, except for root PFS, for more information see
.Xr HAMMER 5 .
.\" ==== pfs-slave ====
.It Cm pfs-slave Ar dirpath Op Ar options
Create a pseudo-filesystem (PFS) inside a
.Nm HAMMER
file system.
Up to 65536 PFSs can be created.
Each PFS uses an independent inode numbering space making it suitable
for replication.
.Pp
The
.Cm pfs-slave
directive creates a PFS that you can use as a mirroring source or target.
You will not be able to access a slave PFS until you have completed the
first mirroring operation with it as the target (its root directory will
not exist until then).
.Pp
Access to the pfs-slave via the special softlink, as described in the
.Sx PFS NOTES
below, allows
.Nm HAMMER
to
dynamically modify the snapshot transaction id by returning a dynamic result
from
.Xr readlink 2
calls.
.Pp
A PFS can only be truly destroyed with the
.Cm pfs-destroy
directive.
Removing the softlink will not destroy the underlying PFS.
.Pp
A PFS can only be created in the root PFS (PFS# 0),
not in a PFS created by
.Cm pfs-master
or
.Cm pfs-slave
(PFS# >0).
.Pp
It is recommended that
.Ar dirpath
is of the form
.Pa <fs>/pfs/<name>
(i.e.\& located in
.Pa pfs
directory at root of
.Nm HAMMER
file system).
.Pp
It is recommended to use a
.Nm null
mount to access a PFS, except for root PFS, for more information see
.Xr HAMMER 5 .
.\" ==== pfs-update ====
.It Cm pfs-update Ar dirpath Op Ar options
Update the configuration parameters for an existing
.Nm HAMMER
file system or pseudo-filesystem.
Options that may be specified:
.Bl -tag -width indent
.It Cm sync-beg-tid= Ns Ar 0x16llx
This is the automatic snapshot access starting transaction id for
mirroring slaves.
This parameter is normally updated automatically by the
.Cm mirror-write
directive.
.Pp
It is important to note that accessing a mirroring slave
with a transaction id greater than the last fully synchronized transaction
id can result in an unreliable snapshot since you will be accessing
data that is still undergoing synchronization.
.Pp
Manually modifying this field is dangerous and can result in a broken mirror.
.It Cm sync-end-tid= Ns Ar 0x16llx
This is the current synchronization point for mirroring slaves.
This parameter is normally updated automatically by the
.Cm mirror-write
directive.
.Pp
Manually modifying this field is dangerous and can result in a broken mirror.
.It Cm shared-uuid= Ns Ar uuid
Set the shared UUID for this file system.
All mirrors must have the same shared UUID.
For safety purposes the
.Cm mirror-write
directives will refuse to operate on a target with a different shared UUID.
.Pp
Changing the shared UUID on an existing, non-empty mirroring target,
including an empty but not completely pruned target,
can lead to corruption of the mirroring target.
.It Cm unique-uuid= Ns Ar uuid
Set the unique UUID for this file system.
This UUID should not be used anywhere else,
even on exact copies of the file system.
.It Cm label= Ns Ar string
Set a descriptive label for this file system.
.It Cm snapshots= Ns Ar string
Specify the snapshots directory which
.Nm
.Cm cleanup
will use to manage this PFS.
.Bl -tag -width indent
.It Nm HAMMER No version 2-
The snapshots directory does not need to be configured for
PFS masters and will default to
.Pa <pfs>/snapshots .
.Pp
PFS slaves are mirroring slaves so you cannot configure a snapshots
directory on the slave itself to be managed by the slave's machine.
In fact, the slave will likely have a
.Pa snapshots
sub-directory mirrored
from the master, but that directory contains the configuration the master
is using for its copy of the file system, not the configuration that we
want to use for our slave.
.Pp
It is recommended that
.Pa <fs>/var/slaves/<name>
be configured for a PFS slave, where
.Pa <fs>
is the base
.Nm HAMMER
file system, and
.Pa <name>
is an appropriate label.
.It Nm HAMMER No version 3+
The snapshots directory does not need to be configured for PFS masters or
slaves.
The snapshots directory defaults to
.Pa /var/hammer/<pfs>
.Pa ( /var/hammer/root
for root mount).
.El
.Pp
You can control snapshot retention on your slave independent of the master.
.It Cm snapshots-clear
Zero out the
.Cm snapshots
directory path for this PFS.
.It Cm prune-min= Ns Ar N Ns Cm d
.It Cm prune-min= Ns Oo Ar N Ns Cm d/ Oc Ns \
Ar hh Ns Op Cm \&: Ns Ar mm Ns Op Cm \&: Ns Ar ss
Set the minimum fine-grained data retention period.
.Nm HAMMER
always retains fine-grained history up to the most recent snapshot.
You can extend the retention period further by specifying a non-zero
pruning minimum.
Any snapshot softlinks within the retention period are ignored
for the purposes of pruning (i.e.\& the fine grained history is retained).
Number of days, hours, minutes and seconds are given as
.Ar N , hh , mm
and
.Ar ss .
.Pp
Because the transaction id in the snapshot softlink cannot be used
to calculate a timestamp,
.Nm HAMMER
uses the earlier of the
.Fa st_ctime
or
.Fa st_mtime
field of the softlink to
determine which snapshots fall within the retention period.
Users must be sure to retain one of these two fields when manipulating
the softlink.
.El
.\" ==== pfs-upgrade ====
.It Cm pfs-upgrade Ar dirpath
Upgrade a PFS from slave to master operation.
The PFS will be rolled back to the current end synchronization transaction id
(removing any partial synchronizations), and will then become writable.
.Pp
.Em WARNING!
.Nm HAMMER
currently supports only single masters and using
this command can easily result in file system corruption
if you don't know what you are doing.
.Pp
This directive will refuse to run if any programs have open descriptors
in the PFS, including programs chdir'd into the PFS.
.\" ==== pfs-downgrade ====
.It Cm pfs-downgrade Ar dirpath
Downgrade a master PFS from master to slave operation.
The PFS becomes read-only and access will be locked to its
.Cm sync-end-tid .
.Pp
This directive will refuse to run if any programs have open descriptors
in the PFS, including programs chdir'd into the PFS.
.\" ==== pfs-destroy ====
.It Cm pfs-destroy Ar dirpath
This permanently destroys a PFS.
.Pp
This directive will refuse to run if any programs have open descriptors
in the PFS, including programs chdir'd into the PFS.
As safety measure the
.Fl y
flag have no effect on this directive.
.\" ==== mirror-read ====
.It Cm mirror-read Ar filesystem Op Ar begin-tid
Generate a mirroring stream to stdout.
The stream ends when the transaction id space has been exhausted.
.Ar filesystem
may be a master or slave PFS.
.\" ==== mirror-read-stream ====
.It Cm mirror-read-stream Ar filesystem Op Ar begin-tid
Generate a mirroring stream to stdout.
Upon completion the stream is paused until new data is synced to the
.Ar filesystem ,
then resumed.
Operation continues until the pipe is broken.
See the
.Cm mirror-stream
command for more details.
.\" ==== mirror-write ====
.It Cm mirror-write Ar filesystem
Take a mirroring stream on stdin.
.Ar filesystem
must be a slave PFS.
.Pp
This command will fail if the
.Cm shared-uuid
configuration field for the two file systems do not match.
See the
.Cm mirror-copy
command for more details.
.Pp
If the target PFS does not exist this command will ask you whether
you want to create a compatible PFS slave for the target or not.
.\" ==== mirror-dump ====
.It Cm mirror-dump
A
.Cm mirror-read
can be piped into a
.Cm mirror-dump
to dump an ASCII representation of the mirroring stream.
.\" ==== mirror-copy ====
.\".It Cm mirror-copy Ar [[user@]host:]filesystem [[user@]host:]filesystem
.It Cm mirror-copy \
Oo Oo Ar user Ns Cm @ Oc Ns Ar host Ns Cm \&: Oc Ns Ar filesystem \
Oo Oo Ar user Ns Cm @ Oc Ns Ar host Ns Cm \&: Oc Ns Ar filesystem
This is a shortcut which pipes a
.Cm mirror-read
command to a
.Cm mirror-write
command.
If a remote host specification is made the program forks a
.Xr ssh 1
and execs the
.Cm mirror-read
and/or
.Cm mirror-write
on the appropriate host.
The source may be a master or slave PFS, and the target must be a slave PFS.
.Pp
This command also establishes full duplex communication and turns on
the 2-way protocol feature
.Fl ( 2 )
which automatically negotiates transaction id
ranges without having to use a cyclefile.
If the operation completes successfully the target PFS's
.Cm sync-end-tid
will be updated.
Note that you must re-chdir into the target PFS to see the updated information.
If you do not you will still be in the previous snapshot.
.Pp
If the target PFS does not exist this command will ask you whether
you want to create a compatible PFS slave for the target or not.
.\" ==== mirror-stream ====
.\".It Cm mirror-stream Ar [[user@]host:]filesystem [[user@]host:]filesystem
.It Cm mirror-stream \
Oo Oo Ar user Ns Cm @ Oc Ns Ar host Ns Cm \&: Oc Ns Ar filesystem \
Oo Oo Ar user Ns Cm @ Oc Ns Ar host Ns Cm \&: Oc Ns Ar filesystem
This is a shortcut which pipes a
.Cm mirror-read-stream
command to a
.Cm mirror-write
command.
This command works similarly to
.Cm mirror-copy
but does not exit after the initial mirroring completes.
The mirroring operation will resume as changes continue to be made to the
source.
The command is commonly used with
.Fl i Ar delay
and
.Fl b Ar bandwidth
options to keep the mirroring target in sync with the source on a continuing
basis.
.Pp
If the pipe is broken the command will automatically retry after sleeping
for a short while.
The time slept will be 15 seconds plus the time given in the
.Fl i
option.
.Pp
This command also detects the initial-mirroring case and spends some
time scanning the B-Tree to find good break points, allowing the initial
bulk mirroring operation to be broken down into 4GB pieces.
This means that the user can kill and restart the operation and it will
not have to start from scratch once it has gotten past the first chunk.
The
.Fl S
option may be used to change the size of pieces and the
.Fl B
option may be used to disable this feature and perform an initial bulk
transfer instead.
.\" ==== version ====
.It Cm version Ar filesystem
This command returns the
.Nm HAMMER
file system version for the specified
.Ar filesystem
as well as the range of versions supported in the kernel.
The
.Fl q
option may be used to remove the summary at the end.
.\" ==== version-upgrade ====
.It Cm version-upgrade Ar filesystem Ar version Op Cm force
Upgrade the
.Nm HAMMER
.Ar filesystem
to the specified
.Ar version .
Once upgraded a file system may not be downgraded.
If you wish to upgrade a file system to a version greater or equal to the
work-in-progress (WIP) version number you must specify the
.Cm force
directive.
Use of WIP versions should be relegated to testing and may require wiping
the file system as development progresses, even though the WIP version might
not change.
.Pp
.Em NOTE!
This command operates on the entire
.Nm HAMMER
file system and is not a per PFS operation.
All PFS's will be affected.
.Bl -tag -width indent
.It 1
.Dx 2.0
default version, first
.Nm HAMMER
release.
.It 2
.Dx 2.3 .
New directory entry layout.
This version is using a new directory hash key.
.It 3
.Dx 2.5 .
New snapshot management, using file system meta-data for saving
configuration file and snapshots (transaction ids etc.).
Also default snapshots directory has changed.
.It 4
.Dx 2.6
default version.
New undo/redo/flush, giving
.Nm HAMMER
a much faster sync and fsync.
.It 5
.Dx 2.9 .
Deduplication support.
.It 6
.Dx 2.9 .
Directory hash ALG1.
Tends to maintain inode number / directory name entry ordering better
for files after minor renaming.
.El
.El
.Sh PSEUDO-FILESYSTEM (PFS) NOTES
The root of a PFS is not hooked into the primary
.Nm HAMMER
file system as a directory.
Instead,
.Nm HAMMER
creates a special softlink called
.Ql @@PFS%05d
(exactly 10 characters long) in the primary
.Nm HAMMER
file system.
.Nm HAMMER
then modifies the contents of the softlink as read by
.Xr readlink 2 ,
and thus what you see with an
.Nm ls
command or if you were to
.Nm cd
into the link.
If the PFS is a master the link reflects the current state of the PFS.
If the PFS is a slave the link reflects the last completed snapshot, and the
contents of the link will change when the next snapshot is completed, and
so forth.
.Pp
The
.Nm
utility employs numerous safeties to reduce user foot-shooting.
The
.Cm mirror-copy
directive requires that the target be configured as a slave and that the
.Cm shared-uuid
field of the mirroring source and target match.
.Sh DOUBLE_BUFFER MODE
There is a limit to the number of vnodes the kernel can cache, and because
file buffers are associated with a vnode the related data cache can get
blown away when operating on large numbers of files even if the system has
sufficient memory to hold the file data.
.Pp
If you turn on
.Nm HAMMER Ns 's
double buffer mode by setting the
.Xr sysctl 8
node
.Va vfs.hammer.double_buffer
to 1
.Nm HAMMER
will cache file data via the block device and copy it into the per-file
buffers as needed.  The data will be double-cached at least until the
buffer cache throws away the file buffer.
This mode is typically used in conjuction with
.Xr swapcache 8
when
.Va vm.swapcache.data_enable
is turned on in order to prevent unnecessary re-caching of file data
due to vnode recycling.
The swapcache will save the cached VM pages related to
.Nm HAMMER Ns 's
block
device (which doesn't recycle unless you umount the filesystem) instead
of the cached VM pages backing the file vnodes.
.Pp
Double buffering should also be turned on if live dedup is enabled via
.Va vfs.hammer.live_dedup .
This is because the live dedup must validate the contents of a potential
duplicate file block and it must run through the block device to do that
and not the file vnode.
If double buffering is not enabled then live dedup will create extra disk
reads to validate potential data duplicates.
.Sh UPGRADE INSTRUCTIONS HAMMER V1 TO V2
This upgrade changes the way directory entries are stored.
It is possible to upgrade a V1 file system to V2 in place, but
directories created prior to the upgrade will continue to use
the old layout.
.Pp
Note that the slave mirroring code in the target kernel had bugs in
V1 which can create an incompatible root directory on the slave.
Do not mix a
.Nm HAMMER
master created after the upgrade with a
.Nm HAMMER
slave created prior to the upgrade.
.Pp
Any directories created after upgrading will use a new layout.
.Sh UPGRADE INSTRUCTIONS HAMMER V2 TO V3
This upgrade adds meta-data elements to the B-Tree.
It is possible to upgrade a V2 file system to V3 in place.
After issuing the upgrade be sure to run a
.Nm
.Cm cleanup
to perform post-upgrade tasks.
.Pp
After making this upgrade running a
.Nm
.Cm cleanup
will move the
.Pa <pfs>/snapshots
directory for each PFS mount into
.Pa /var/hammer/<pfs> .
A
.Nm HAMMER
root mount will migrate
.Pa /snapshots
into
.Pa /var/hammer/root .
Migration occurs only once and only if you have not specified
a snapshots directory in the PFS configuration.
If you have specified a snapshots directory in the PFS configuration no
automatic migration will occur.
.Pp
For slaves, if you desire, you can migrate your snapshots
config to the new location manually and then clear the
snapshot directory configuration in the slave PFS.
The new snapshots hierarchy is designed to work with
both master and slave PFSs equally well.
.Pp
In addition, the old config file will be moved to file system meta-data,
editable via the new
.Nm
.Cm viconfig
directive.
The old config file will be deleted.
Migration occurs only once.
.Pp
The V3 file system has new
.Cm snap*
directives for creating snapshots.
All snapshot directives, including the original, will create
meta-data entries for the snapshots and the pruning code will
automatically incorporate these entries into its list and
expire them the same way it expires softlinks.
If you by accident blow away your snapshot softlinks you can use the
.Cm snapls
directive to get a definitive list from the file system meta-data and
regenerate them from that list.
.Pp
.Em WARNING!
If you are using
.Nm
to backup file systems your scripts may be using the
.Cm synctid
directive to generate transaction ids.
This directive does not create a snapshot.
You will have to modify your scripts to use the
.Cm snapq
directive to generate the linkbuf for the softlink you create, or
use one of the other
.Cm snap*
directives.
The older
.Cm snapshot
directive will continue to work as expected and in V3 it will also
record the snapshot transaction id in file system meta-data.
You may also want to make use of the new
.Ar note
tag for the meta-data.
.Pp
.Em WARNING!
If you used to remove snapshot softlinks with
.Nm rm
you should probably start using the
.Cm snaprm
directive instead to also remove the related meta-data.
The pruning code scans the meta-data so just removing the
softlink is not sufficient.
.Sh UPGRADE INSTRUCTIONS HAMMER V3 TO V4
This upgrade changes undo/flush, giving faster sync.
It is possible to upgrade a V3 file system to V4 in place.
This upgrade reformats the UNDO/REDO FIFO (typically 1GB),
so upgrade might take a minute or two depending.
.Pp
Version 4 allows the UNDO/REDO FIFO to be flushed without also having
to flush the volume header, removing 2 of the 4 disk syncs typically
required for an
.Fn fsync
and removing 1 of the 2 disk syncs typically
required for a flush sequence.
Version 4 also implements the REDO log (see
.Sx FSYNC FLUSH MODES
below) which is capable
of fsync()ing with either one disk flush or zero disk flushes.
.Sh UPGRADE INSTRUCTIONS HAMMER V4 TO V5
This upgrade brings in deduplication support.
It is possible to upgrade a V4 file system to V5 in place.
Technically it makes the layer2
.Va bytes_free
field a signed value instead of unsigned, allowing it to go negative.
A version 5 filesystem is required for dedup operation.
.Sh UPGRADE INSTRUCTIONS HAMMER V5 TO V6
It is possible to upgrade a V5 file system to V6 in place.
.Sh FSYNC FLUSH MODES
.Nm HAMMER
implements five different fsync flush modes via the
.Va vfs.hammer.fsync_mode
sysctl, for
.Nm HAMMER
version 4+ file systems.
.Pp
As of
.Dx 2.6
fsync mode 3 is set by default.
REDO operation and recovery is enabled by default.
.Bl -tag -width indent
.It mode 0
Full synchronous fsync semantics without REDO.
.Pp
.Nm HAMMER
will not generate REDOs.
A
.Fn fsync
will completely sync
the data and meta-data and double-flush the FIFO, including
issuing two disk synchronization commands.
The data is guaranteed
to be on the media as of when
.Fn fsync
returns.
Needless to say, this is slow.
.It mode 1
Relaxed asynchronous fsync semantics without REDO.
.Pp
This mode works the same as mode 0 except the last disk synchronization
command is not issued.
It is faster than mode 0 but not even remotely
close to the speed you get with mode 2 or mode 3.
.Pp
Note that there is no chance of meta-data corruption when using this
mode, it simply means that the data you wrote and then
.Fn fsync Ns 'd
might not have made it to the media if the storage system crashes at a bad
time.
.Pp
.It mode 2
Full synchronous fsync semantics using REDO.
NOTE: If not running a
.Nm HAMMER
version 4 filesystem or later mode 0 is silently used.
.Pp
.Nm HAMMER
will generate REDOs in the UNDO/REDO FIFO based on a heuristic.
If this is sufficient to satisfy the
.Fn fsync
operation the blocks will be written out and
.Nm HAMMER
will wait for the I/Os to complete,
and then followup with a disk sync command to guarantee the data
is on the media before returning.
This is slower than mode 3 and can result in significant disk or
SSDs overheads, though not as bad as mode 0 or mode 1.
.Pp
.It mode 3
Relaxed asynchronous fsync semantics using REDO.
NOTE: If not running a
.Nm HAMMER
version 4 filesystem or later mode 1 is silently used.
.Pp
.Nm HAMMER
will generate REDOs in the UNDO/REDO FIFO based on a heuristic.
If this is sufficient to satisfy the
.Fn fsync
operation the blocks
will be written out and
.Nm HAMMER
will wait for the I/Os to complete,
but will
.Em NOT
issue a disk synchronization command.
.Pp
Note that there is no chance of meta-data corruption when using this
mode, it simply means that the data you wrote and then
.Fn fsync Ns 'd
might
not have made it to the media if the storage system crashes at a bad
time.
.Pp
This mode is the fastest production fsyncing mode available.
This mode is equivalent to how the UFS fsync in the
.Bx Ns s
operates.
.Pp
.It mode 4
fsync is ignored.
.Pp
Calls to
.Fn fsync
will be ignored.
This mode is primarily designed
for testing and should not be used on a production system.
.El
.Sh RESTORING FROM A SNAPSHOT BACKUP
You restore a snapshot by copying it over to live, but there is a caveat.
The mtime and atime fields for files accessed via a snapshot is locked
to the ctime in order to keep the snapshot consistent, because neither
mtime nor atime changes roll any history.
.Pp
In order to avoid unnecessary copying it is recommended that you use
.Nm cpdup
.Fl VV
.Fl v
when doing the copyback.
Also make sure you traverse the snapshot softlink by appending a ".",
as in "<snapshotpath>/.", and you match up the directory properly.
.Sh RESTORING A PFS FROM A MIRROR
A PFS can be restored from a mirror with
.Cm mirror-copy .
.Cm config
data must be copied separately.
At last the PFS can be upgraded to master using
.Cm pfs-upgrade .
.Pp
It is not possible to restore the root PFS (PFS# 0) by using mirroring,
as the root PFS is always a master PFS.
A normal copy (e.g.\& using
.Xr cpdup 1 )
must be done, ignoring history.
If history is important, old root PFS can me restored to a new PFS, and
important directories/files can be
.Nm null
mounted to the new PFS.
.Sh EXIT STATUS
.Ex -std
.Sh ENVIRONMENT
If the following environment variables exist, they will be used by:
.Bl -tag -width ".Ev EDITOR"
.It Ev EDITOR
The editor program specified in the variable
.Ev EDITOR
will be invoked instead of the default editor, which is
.Xr vi 1 .
.It Ev VISUAL
Same effect as
.Ev EDITOR
variable.
.El
.Sh FILES
.Bl -tag -width ".It Pa <fs>/var/slaves/<name>" -compact
.It Pa <pfs>/snapshots
default per PFS snapshots directory
.Nm ( HAMMER
VERSION 2-)
.It Pa /var/hammer/<pfs>
default per PFS snapshots directory (not root)
.Nm ( HAMMER
VERSION 3+)
.It Pa /var/hammer/root
default snapshots directory for root directory
.Nm ( HAMMER
VERSION 3+)
.It Pa <snapshots>/config
per PFS
.Nm
.Cm cleanup
configuration file
.Nm ( HAMMER
VERSION 2-)
.It Pa <fs>/var/slaves/<name>
recommended slave PFS snapshots directory
.Nm ( HAMMER
VERSION 2-)
.It Pa <fs>/pfs
recommended PFS directory
.El
.\".Sh EXAMPLES
.Sh SEE ALSO
.Xr ssh 1 ,
.Xr undo 1 ,
.Xr HAMMER 5 ,
.Xr periodic.conf 5 ,
.Xr loader 8 ,
.Xr mount_hammer 8 ,
.Xr mount_null 8 ,
.Xr newfs_hammer 8 ,
.Xr swapcache 8 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
utility first appeared in
.Dx 1.11 .
.Sh AUTHORS
.An Matthew Dillon Aq dillon@backplane.com