Netatalk Frequently Asked Questions
-($Id: FAQ,v 1.8 2002-02-01 06:15:34 srittau Exp $)
+($Id: FAQ,v 1.9 2002-10-04 23:38:44 srittau Exp $)
-----------------------------------------------------------------------------
of these problems. This code, along with the CNID code, was still being
worked out in Pre7. The cnid/db3 flags also go along with this:
--with-db3=PATH specify path to Berkeley DB3 installation
- --with-did=[scheme] set DID scheme (last,mtab,cnid)
+ --with-did=[scheme] set DID scheme (cnid,last)
(For more information on CNID, see the README.cnid file [may not exist yet],
into which I just copied wholesale Joe's comments on what he did with
--with-db3=PATH: specify path to Berkeley DB3 installation,
---with-did=[scheme]: set DID scheme (last,mtab,cnid),
+--with-did=[scheme]: set DID scheme (cnid,last),
+ CNID is a new scheme using Berkeley DB3 files to store a
+ per-volume cnid database persistently.
+
The last DID scheme recreates version 37b behaviour where
directory id's are incrementally calculated versus the new
hash method. Unfortunately for machines that have a lot of
devices, and/or a lot of inodes the hash can fail with
multiple directories resolving to the same DID.
- cnid is a new and experimental scheme using Berkeley DB3 files
- to store a per-volume cnid database persistently.
-
--with-message-dir=path: path to message files [default is DISABLED],
--enable-srvloc: Turn on Server Location Protocol support,
# Makefile.am for INSTALL/
EXTRA_DIST = \
- CONFIGURE \
- COPYRIGHT.mtab \
- DEVELOPER \
- INSTALL \
- FAQ \
- README.hidden-items \
- README.mtab \
- README.mtab.distribution \
+ CONFIGURE \
+ COPYRIGHT.mtab \
+ DEVELOPER \
+ INSTALL \
+ FAQ \
+ README.hidden-items \
README.platforms \
README.veto
So how does netatalk approach the problem?
-Netatalk has 4 different methods of allocating IDs: last, hash, mtab or cnid.
+Netatalk has two different methods of allocating IDs: last and cnid.
DID = last.
the mac features which don't rely heavily on IDs it works fine. If you try
to use IDs much, things break.
-DID = hash.
-
-This uses inodes to identify files. An inode is a unique number which
-is the equivalent to ID on ext2 (and 3) file systems. However, an inode is
-significanlty bigger than an ID, so the number needs to be hashed to make it
-smaller. Unfortunately this means it is nolonger unique, so you can get 2
-files with the same ID.
-
-DID = mtab.
-
-This is similar to hash, but better. This is not available on all operating
-systems. See README.mtab for more details.
-
DID = cnid.
This uses a Berkeley 3 database to store and maintain a directory of IDs
+++ /dev/null
-The afpd.mtab file
-
- This file usually lives in /etc/atalk/afpd.mtab, and contains
-information describing each mounted partition that should be accessible
-via afpd, in a format similar to /etc/mtab or /etc/fstab, one entry per
-line. Each entry consists of three values: an integer index, a disk
-device, and the mount point for that disk device, separated by arbitrary
-amounts of whitespace. Here is an example, created automatically by the
-afpd-mtab.pl script from the list of mounted partitions in /etc/mtab
-[this works on Linux, YMMV]:
-
- rgr> ./afpd-mtab.pl < /etc/mtab
- # afpd.mtab, generated by afpd-mtab.pl on Mon Apr 9 18:15:15 EDT 2001
- 1 /dev/hda8 /
- 2 /dev/hda12 /var
- 3 /dev/hda11 /usr
- 4 /dev/hda10 /usr/local
- 5 /dev/hda9 /home
- rgr>
-
-The index values must be small positive integers, and must be unique to
-each device. They need not be dense; in fact, if a partition is ever
-retired, a gap should be left so that dangling aliases do not point to
-files on a different device. The index 0 is reserved. The mount point
-is not actually used currently, thought it is useful for reference
-purposes; in any case, it must be present.
-
-How afpd.mtab is used
-
- The index values are used in constructing 32-bit unique directory
-identifiers (DIDs), which the client uses to make aliases to netatalk
-shares. Because of these aliases, it is important that DIDs be
-
- 1. Unique, to be sure that the client and server are really talking
-about the same directories; and
-
- 2. Persistent indefinitely despite server restarts & reboots, so
-that aliases don't become stale.
-
-In Unix, files can be uniquely and persistently identified by (device,
-inode) pairs, where devices are identified by a pair of (major, minor)
-device numbers. But because the Apple File Protocol only allocates 32
-bits for the whole DID value, and the Unix device address space is
-generally huge but usually very sparse, afpd must go through some
-contortions to compress (major, minor, inode) triples into 32 bits. The
-inode value is dense and therefore not compressible, but the device
-space is that used in a given installation can usually be compressed
-into 2 to 4 bits. While this compression could be done on the fly, by
-picking an arbitrary encoding at startup, that would fail persistence
-requirement, especially when restarting after adding new disks to the
-system. Hence the introduction of the afpd.mtab file for the sake of
-persistence.
-
- Upon reading this file at startup, afpd arranges to use just enough
-bits for the device subfield to encode all devices in the afpd.mtab
-file, leaving the rest for the inode. If 4 bits are required for the
-device, allowing indices from 1 through 15, then 28 are left for the
-inode, leaving room for 256Mi files on each device. On an ext2 file
-system made with default options, where there are about 3.9 blocks per
-inode, that allows partitions of nearly a terabyte. By the time
-netatalk needs to deal with such partitions, AFP will probably be
-obsolete. If you are unlucky enough to be ahead of the curve, then you
-probably have lots of large files to store (e.g. sound, images, video),
-and you should consider raising the blocks-per-inode ratio anyway.
-
- Note that afpd *only* reads the afpd.mtab file at startup, so to get
-it to recognize a new partition, one must (1) restart afpd, and (2)
-notify all clients that they need to close and reopen their server
-connections before they access the new volume.
-
-Maintaining afpd.mtab
-
- It is expected that once afpd.mtab is created initially, possibly
-using the afpd-mtab.pl script provided, it will then be maintained by
-hand so as to preserve the mapping where possible. For example,
-repartitioning may cause some of the partition numbers and hence the
-device names to change, but the underlying inode->file mapping is the
-same, so the association of index to mount point should be maintained.
-On the other hand, if the contents of a partition are moved to another
-partition, the inode numbers will all change (unless steps can be taken
-to preserve them). In that case, best practice would be to "retire" the
-old index and choose a new one; otherwise, aliases will appear to be
-remapped at random, possibly with catastrophic results.
-
+++ /dev/null
-About this directory
-
- This directory contains I hack I wrote to explore a way to fix the
-netatalk persistent DID problem. A survey of the contents:
-
- README.mtab -- notes for implementors and sysadmins, on why there is
-an extra configuration file, and how to maintain it. If adopted, this
-should become part of the distribution documentation. Much more could
-be said.
-
- afpd-mtab.pl -- perl script that acts as a filter, turning /etc/mtab
-into an initial afpd.mtab version. (Only tested on RH Linux.)
-
- afpd.mtab.* -- testing versions produced on my system. "make test"
-tries to produce afpd.mtab automatically; if this doesn't work on your
-configuration, you could try editing one of these to match your
-partition structure.
-
- parse-mtab.c, parse-mtab.h -- source intended to be integrated into
-the etc/afpd/ code.
-
- test-parse-mtab.c -- test driver for parse-mtab.c, reads the file and
-produces CNID values for other files and directories given on the
-command line. Change top_srcdir in the makefile and do "make test" in
-order to compile & run this.
-
-Notes:
-
- * I have only ext2fs partitions on a single local disk, so I haven't
-been able to do very broad testing, even on Linux. This is part of why
-I'm too chicken to try to paste this into the netatalk source. ;-}
-
- * If there is no entry in the table, I just use the reserved 0 device
-index. This seems like asking for trouble, so a better policy would be
-to make absent devices unavailable (you probably don't want users poking
-around in /var or /usr anyway), but that requires a more global
-implementation.
-
- * The way it's now written, afpd is expected to fail to start up if
-/etc/atalk/afpd.mtab is not found. That may not be optimal, but even if
-it is, it requires some extra setup instructions.
-
- * I'm assuming the high order bit need no longer be reserved for the
-file/directory flag, but I have no idea whether that will work. If not,
-it's easy to change.
-
- * It would be good if, on startup, afpd would warn about partitions
-in the table that have more inodes than can be represented in the
-available number of bits.
-
- * If partitions were to use a bit encoding that reversed the bits in
-the index (i.e. putting the LSB "first" as written), then the device MSB
-would be adjacent to the inode MSB. That would produce upward-
-compatible DIDs if the number of device bits needed to increase. The
-encoding is precomputed anyway, so it's not a speed concern. Better
-still, by carefully choosing small indices for large-inode devices and
-avoiding intermediate-sized indices, one could effectively set the
-device/inode boundary independently for each device. This would allow a
-sysadmin to come close to using all of the theoretical 32-bit DID-space.
-(Implementation is left as an excercise. ;-)
-
projects / netatalk.git / commitdiff