sbin/jscan/jscan.8

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.Dd February 8, 2009
.Dt JSCAN 8
.Os
.Sh NAME
.Nm jscan
.Nd journal file processing program
.Sh SYNOPSIS
.Nm
.Op Fl 2dfuvF
.Op Fl c Ar count[k,m,g,t]
.Op Fl D Ar directory
.Op Fl m Ar mirror_transid_file/none
.Op Fl o/O Ar output_transid_file/none
.Op Fl s Ar size[k,m,g,t]
.Op Fl w/W Ar journal_prefix
.Op Ar journal_prefix/file
.Sh DESCRIPTION
The
.Nm
utility scans journal file or input stream for the purposes of debugging
dumps, restoration, undo, mirroring, and other journaling features.
.Bl -tag -width indent
.It Fl 2
Implement the full-duplex acknowledgement protocol on the input descriptor.
Note that shell pipes are full-duplex and can be used with this option.
.It Fl c Ar count
Specify the number of transaction records which should be scanned, then exit.
This option is typically used along with
.Fl m
to limit the amount of work that
.Nm
does, giving you the ability to incrementally run a mirror forwards or
backwards.  It is not usually used when piping in a live journal, but it
can be.
.It Fl d
Display the contents of the journaling file or stream in a human readable
format on stderr.  Note that stdout is used only for
.Fl o .
.It Fl f
.Nm
will sleep for 5 seconds and loop when it hits EOF on file or prefix
set input rather than exit.  This option is typically used when running
on an input file or prefix set which is live (being written to by
another
.Nm
instance).
.It Fl D Ar directory
Specify the base directory for the mirroring option.
.It Fl m Ar mirror_transid_file/none
Generate a mirror in the directory specified by
.Fl D
or, if not specified, the current directory.
The
.Ar mirror_transid_file
will be used to track the transaction id representing the current
synchronization point for the mirror.  The keyword
.Ar none
may be specified if no tracking file is desired.  However, if no tracking
file is specified it will not be possible to roll the mirror forwards or
backwards or restart the journaling stream being used to generate the mirror.
.Pp
It is important to note that journaling streams can contain meta-transactions
representing huge, multi-gigabyte operations.  If the journaling data is
not being recorded to regular files via
.Fl w/W
it is possible that
.Nm
could run itself out of memory trying to record the meta-transactions.
In addition, the mirror would not be restartable.  If the journaling data
is being recorded via
.Fl w/W
and a mirroring transaction id file is being kept, the mirror can be
restarted.
.Pp
While it is possible to run a journaling stream directly into a mirror,
it is more typical to file the jornaling stream with
.Fl w
and catch the mirror up as a batch job with the journaling file set prefix
specified as the input every so often.  This way the system operator can
use other
.Nm
commands to, for example, run a mirror backwards and forwards in time.
.It Fl o/O Ar output_transid_file/none
Generate a journaling stream on stdout using the specified file to track
the transaction id to help with restarts.
The
.Fl o
option indicates a half-duplex output stream while the
.Fl O
option indicates a full-duplex (ACK protocol) output stream.
.Pp
This option is not really designed to output to regular files because it
does NOT necessarily weed out duplicate records.  When both the input
stream and output stream are full-duplex and
.Fl w/W
is not specified,
.Nm
acts as a stateless transceiver and the input stream is not acked until
an ack is received from the output stream.
.Pp
This option is most typically used in conjunction with
.Fl w/W .
In this case the ACK protocol is handled independently for the input side
and the output side uses the journaling data recorded by
.Fl w/W
as a buffer.
.Pp
In half-duplex output mode the output transaction id file is updated
after a raw transaction record has been successfully written to stdout.
In full-duplex output mode the file is only updated with ACK data returned
on the stdout descriptor.
.Pp
As with the
.Fl m
option, you can combine
.Fl o
in a journaling pipe with other options, but if you are trying to use it
as a buffer it may be better to have it separately pull its data off of
a journaling file set generated via
.Fl w .
.It Fl s Ar size
Change the size limit for rotating files created via
.Fl w .
The default is 100M.  Values are in bytes or may be suffixed with k,
m, or g.
If a raw transaction causes the file's size limit to be exceeded, a new file
will be created.  If a raw transaction is, in-whole, larger than the file's
size limit, the raw transaction will still be fully written to the file before
a new file is created.  Raw transactions are typically limited to the size
of the source system's memory FIFO.  This option is typically used to size
journaling files to fit onto the appropriate backup media or to provide
bite-sized chunks for other programs to ingest.
.Pp
When restarting a journal, a new sequence number will always be chosen for
the resumption of data recording.  No existing file will be appended to when
.Nm
is reinvoked.
.It Fl u
Will cause the journal to be scanned backwards (requires seekable media).
Transactions will be dumped in reverse order.  If mirroring, the UNDO
data will be executed.  If not specified, 1 hour's worth of data will be
undone.  Can only be used with a journaling file or journaling prefix
as the input.
.It Fl v
Increase verbosity on stderr.  This option is primarily used for debugging.
.It Fl w Ar prefix
The received journaling stream is recorded in journaling files named
.Ar <prefix>.<seq>
and the current transaction id is tracked in a file named
.Ar <prefix>.transid .
A journaling file is closed out and a new file with the next sequence
number is created once the file surpasses 100MB.
.Pp
This option is robust across restarts.  The current transaction id
will be read and the input stream will be skipped until it is reached.
If the input is a journaling file or prefix set,
.Nm
will be able to quickly seek to the restart point.
.Pp
NOTE: If
you are generating a mirror with the same command via
.Fl m ,
and the journaling data input is a stream rather than a file or prefix
set, you must use
.Fl w/W
if you want the mirror to be restartable.  This is because while we can
pick up the transaction id where we left off, that raw transaction id may
have cut a larger meta-transaction in half and the mirroring code will
not be able to access the whole of the transaction unless it has a file
or prefix set to work with.
.It Fl W Ar prefix
Similar to
.Fl w
except that the journaling files created are strictly temporary and will
be deleted once they exceed the size limit AND the related meta-transactions
have been completed.
.Pp
If combined with
.Fl m ,
the meta-transactions are considered to be completed only when the mirror
finishes executing them.  It is possible for several sequence number files
to build if a particularly large meta-transaction is coming down the pipe.
.Pp
If combined with
.Fl o/O ,
the meta-transactions are considered to be completed when the data has
been successfully written out to the pipe in half duplex mode, or when
the ACK has been received in full-duplex mode.
.Pp
If both
.Fl m
and
.Fl w/W
is used, the journaling data files are only deleted when both actions
no longer need the data.
.It Fl F
Forces
.Nm
to
.Fn fsync
after updating a journaling file prior to acknowledging the
data or updating a transaction-id-tracking file.  If specified twice,
.Nm
will also
.Fn fsync
after updating the transaction-id-tracking file.
.It Ar journal_prefix/file
Specify the input to
.Nm .
This can be a journaling file set prefix
or it can be a plain file.  If no input file is specified, stdin is
assumed.  Note that when generating a mirror from a stdin stream, the
mirror will not be restartable unless
.Fl w/W
is also used.
.El
.Sh OPERATIONAL NOTES
It is often important to be able to quickly stage journaled data through
a dedicated backup machine on a LAN.  There are several places where data
can be buffered and staged out.
.Pp
The machine generating the journal typically buffers several megabytes of
journal data in the kernel.  This local machine can pipe that data to
.Nm
or some other locally run program to add another buffering stage, or you
can directly attach a TCP connection to the kernel's journaling output.
.Pp
The LAN backup box typically buffers gigabytes worth of data by running
multiple jscans.  The
.Nm
on the receiving end of the TCP or pipe (for
example, via ssh) typically records the data via the
.Fl w
option, and then runs other
.Nm
programs from scripts or cron to take that data and copy it to your
off-site backup machine.  Other
.Nm
programs may use the same data
set to generate mirrors or other backup streams.
.Pp
It should be noted that if
.Fl w/W
is specified, both mirroring mode and output mode will internally
fork the program once the appropriate synchronization point has been reached,
effectively decoupling their operation, and read all of their data via
the journaling files written out by the master program.  In particular,
blockages in the mirroring and output code will not affect our ability
to buffer the journaling input data via
.Fl w/W .
If
.Fl w/W
is not specified then neither the mirroring or output modes will fork.  Under
these conditions, if the input is a stream rather than a file
.Nm
will be forced to buffer meta-transactions (for mirroring) entirely in
memory, which could present a serious problem since a single meta-transaction
can exceed a gigabyte (e.g. if someone were to do a single
.Fn write
system call writing a gigabyte all in one go).
.Sh SEE ALSO
.Xr mountctl 8
.Sh CAVEATS
This utility is currently under construction and not all features have been
implemented yet.
In fact, most have not.
.Sh HISTORY
The
.Nm
utility first appeared in
.Dx 1.3 .