1# This file is README.autofs 2# am-utils-6.1 3# Erez Zadok <ezk AT cs.columbia.edu> a.k.a. "Darth Autoconf" 4# 5# modified by 6# Ion Badulescu <ib42 At cs.columbia.edu> a.k.a. "The Autofs Master" 7 8** General notes about the autofs support in am-utils 9 10- The autofs code in am-utils is gamma quality for Linux, beta quality for 11 Solaris 2.5+ and non-working for all the other systems. 12 13- Link, lofs, ufs, nfs, nfsl, and auto mounts were tested and work properly; 14 the others should work, but were not tested. 15 16* Caveats: 17 18- [this applies to Solaris/IRIX 6/HP-UX/AIX] Amd acts as *both* automountd and 19automount. There's no way to distinguish between the two. When amd starts, 20it first registers itself as an autofs server (automountd's job), then 21parses its own maps, and decides which autofs-type mounts to make 22(automount's job). After the autofs mounts are made, amd listens for 23requests from the kernel-based autofs, and acts upon them. Since there can 24be only one autofs listener on a system, this means that automountd and amd 25cannot run at the same time; nor can two amd's run at the same time if they 26are both using autofs mounts. 27 28- Linux support is available and fairly stable. Solaris 2.5+ support is 29newer and less tested, but seems pretty stable as well. IRIX 6 and HP-UX 30autofs support will probably be very easy once we get Solaris 2.5 to work, 31as they use the same protocol and almost identical data structures. AIX 32seems to be using the Solaris 2.5 protocol as well, but they don't provide 33any headers or documentation so getting autofs to work will be tricky at best. 34 35- Killing amd can become a problem if there are active mounts. Since mounts 36are done "in place", we can't just unmount our mount points and go away. For 37now, amd simply tells the kernel that it is dying; we need to think this 38further. It would be nice to "take over" the old mountpoints, there is 39support for this in Solaris (due to is RPC nature) and is easy to add to 40the Linux kernel (an ioctl on the mountpoint). 41 42- The Solaris 2.5 implementation of the autofs v1 protocol is NON-REENTRANT, 43and ignoring this limitation results in a DEADLOCK between the kernel and 44the daemon. This is a serious problem, although only for the lofs and link 45mount types. In other words, suppose the daemon is trying a lofs/link mount 46whose destination crosses another autofs mountpoint. If that mountpoint is 47not yet mounted, it will trigger another autofs lookup which will deadlock 48inside the kernel -- because the kernel is waiting for the previous request 49to return. This is not even something specific to amd, Sun's own automountd 50has the exact same problem. Ctrl-C breaks the deadlock, so it's not fatal, 51but the lofs/link entry is in effect unusable. 52 53- Solaris 2.8 (at least) doesn't like getting to get out-of-order replies to 54mount requests, and ignores the late ones. It's only a minor annoyance (read 55delay), because the RPC is retried by the kernel and it succeeds the second 56time around. 57 58* Solaris: 59 60- Amd w/ autofs mounts will fight over the listener port with Sun's 61 automountd, so running both simultaneously is a really bad idea. 62 63- Browsable_dirs is possible and implemented for Solaris 2.6+. 64 65- Direct mounts are implemented and work correctly. 66 67- Host maps are implemented and work correctly. 68 69- On Solaris 2.6+, symlinks over autofs are inefficient. The kernel seems to 70 time them out immediately after receiving them, so each access to a 71 symlink causes a call to userspace. Needless to say, this negates the 72 whole point of using autofs. Automountd seems to always use lofs mounts 73 instead of symlinks, we do the same unless the admin requests 74 "autofs_use_lofs = no" in amd.conf. 75 76- Solaris 2.5/2.5.1 do not support symlinks over autofs, so links are always 77 mounted as lofs mounts, subject to the limitation described in the caveats 78 section above. 79 80- Restarting autofs mounts is possible, but not yet implemented. 81 82* Linux: 83 84- Amd should work fine even when the Linux automounter is running, the 85 mechanism being used prevents any kind of (evil) interaction between them. 86 87- Browsing is not available if autofs support is used, due to limitations in 88 the kernel<->daemon protocol used by Linux 2.2 and 2.4. Only already-mounted 89 nodes will appear in the autofs directory, and this is implemented entirely 90 in the kernel. 91 92- Host maps are supported with all autofs versions. 93 94- Direct maps cannot be supported since there is no kernel support for them; 95 we might be able to get something eventually, but don't hold your breath. 96 If anything, we may do it using a regular NFS mountpoint and bind-mount on 97 top of it. 98 99- Inherit doesn't make much sense because we can't restart a hung autofs 100 mount point, due to kernel limitations. This needs to be fixed in the 101 Linux kernel; it's not particularly difficult, and we might provide a 102 patch at some point. 103 104- Link (and lofs) mounts will use the new bind-mount support in Linux 105 2.4+. No more symlinks! And /bin/pwd works great too. 106 107- Auto maps are actually mounted as a separate autofs filesystem. Since each 108 autofs filesystem consumes 2 file descriptors, and amd has at most 1024 109 file descriptors available, there can be at most 512 of them mounted at 110 the same time. Because of this, extensive use of auto maps is discouraged 111 when using autofs on Linux. 112 113* amd.conf requirements: 114 115To tell amd to use an autofs-style mount point and mounts for a map, add 116 117 mount_type = autofs 118 119either to the global section, or to the sections of the individual maps you 120selected. Mixing autofs and normal amd mount points in the same amd.conf 121file *does* work. 122 123* Map changes: 124 125No changes are necessary. If a map is marked as autofs in amd.conf, mounts 126are done "in place" and the "fs" parameter is ignored most of the time. 127 128* Todo: 129 130We are looking for volunteers to improve the autofs code! 131 132(1) These fixes are needed: 133 134- sublinks are broken 135 136- when the mount type is 'link', transparently translate it into a loopback 137file system mount (lofs), that would mount in place, rather than supply a 138symlink, at least on systems whose autofs doesn't support symlinks. Linux 139does support symlinks, Solaris 2.6+ does too, but Solaris 2.5/2.5.1 doesn't 140and neither does IRIX 6. Moreover, Sun's automountd always uses lofs for 141link mounts, even on 2.6+, because symlinks are not cached in the kernel and 142thus are not particularly efficient. [done] 143 144- complain if certain incompatible options (autofs and ...) are used. Direct 145maps on Linux is one such case of incompatible options. Browsable_dirs on 146Linux is another such case. 147 148- if amd is killed or dies, the autofs mounts will remain intact as required 149(your system is _not_ hung, yay!). However, if you restart amd, it will not 150correctly 'restart' the autofs mounts as the Sun automounter does. Rather, 151it might cause another mount to happen, which leaves your /etc/mnttab 152cluttered with older mounts entries which cannot be unmounted. It might also 153just pretend everything is ok, when in fact it isn't. 154 155(2) Code expansion: 156 157- [Solaris only] implement the sockets version of amu_get_autofs_address() 158and create_autofs_service(), in conf/transp/transp_sockets.c. Not sure if 159it's necessary, Solaris it still biased towards TLI/STREAMS in 160userspace. [mostly done, untested, might be used on AIX 5.x+ and IRIX 6] 161 162- Implement the restarting of autofs mount points. This is already doable on 163Solaris; on Linux, the kernel needs to be patched to allow it. 164 165(3) Testing and porting to other systems: 166 167- nothing has been tested on IRIX 6, which reportedly has a similarly 168functioning autofs to Solaris 2.5. The code compiles, but has not been run 169yet and is most likely broken. 170 171- support for Linux autofs is stable, we need testers! 172 173- support for Solaris 2.6+ is pretty stable, so we need testers for it, too! 174 175- we did not test any version of Solaris on x86. It will probably work, but 176you have been warned. Testers are welcome. 177