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111 lines
4.4 KiB
Plaintext
111 lines
4.4 KiB
Plaintext
$FreeBSD$
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Soft Updates Status
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As is detailed in the operational information below, snapshots
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are definitely alpha-test code and are NOT yet ready for production
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use. Much remains to be done to make them really useful, but I
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wanted to let folks get a chance to try it out and start reporting
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bugs and other shortcomings. Such reports should be sent to
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Kirk McKusick <mckusick@mckusick.com>.
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Snapshot Copyright Restrictions
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Snapshots have been introduced to FreeBSD with a `Berkeley-style'
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copyright. The file implementing snapshots resides in the sys/ufs/ffs
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directory and is compiled into the generic kernel by default.
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Using Snapshots
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To create a snapshot of your /var filesystem, run the command:
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mount -u -o snapshot /var/snapshot/snap1 /var
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This command will take a snapshot of your /var filesystem and
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leave it in the file /var/snapshot/snap1. Note that snapshot
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files must be created in the filesystem that is being snapshotted.
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I use the convention of putting a `snapshot' directory at the
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root of each filesystem into which I can place snapshots.
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You may create up to 20 snapshots per filesystem. Active snapshots
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are recorded in the superblock, so they persist across unmount
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and remount operations and across system reboots. When you
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are done with a snapshot, it can be removed with the `rm'
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command. Snapshots may be removed in any order, however you
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may not get back all the space contained in the snapshot as
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another snapshot may claim some of the blocks that it is releasing.
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Note that the `schg' flag is set on snapshots to ensure that
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not even the root user can write to them. The unlink command
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makes an exception for snapshot files in that it allows them
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to be removed even though they have the `schg' flag set, so it
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is not necessary to clear the `schg' flag before removing a
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snapshot file.
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Once you have taken a snapshot, there are three interesting
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things that you can do with it:
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1) Run fsck on the snapshot file. Assuming that the filesystem
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was clean when it was mounted, you should always get a clean
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(and unchanging) result from running fsck on the snapshot.
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If you are running with soft updates and rebooted after a
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crash without cleaning up the filesystem, then fsck of the
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snapshot may find missing blocks and inodes or inodes with
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link counts that are too high. I have not yet added the
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system calls to allow fsck to add these missing resources
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back to the filesystem - that will be added once the basic
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snapshot code is working properly. So, view those reports
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as informational for now.
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2) Run dump on the snapshot. You will get a dump that is
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consistent with the filesystem as of the timestamp of the
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snapshot.
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3) Mount the snapshot as a frozen image of the filesystem.
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To mount the snapshot /var/snapshot/snap1:
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mdconfig -a -t vnode -f /var/snapshot/snap1 -u 4
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mount -r /dev/md4 /mnt
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You can now cruise around your frozen /var filesystem
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at /mnt. Everything will be in the same state that it
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was at the time the snapshot was taken. The one exception
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is that any earlier snapshots will appear as zero length
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files. When you are done with the mounted snapshot:
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umount /mnt
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mdconfig -d -u 4
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Note that under some circumstances, the process accessing
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the frozen filesystem may deadlock. I am aware of this
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problem, but the solution is not simple. It requires
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using buffer read locks rather than exclusive locks when
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traversing the inode indirect blocks. Until this problem
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is fixed, you should avoid putting mounted snapshots into
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production.
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Performance
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It takes about 30 seconds to create a snapshot of an 8Gb filesystem.
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Of that time 25 seconds is spent in preparation; filesystem activity
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is only suspended for the final 5 seconds of that period. Snapshot
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removal of an 8Gb filesystem takes about two minutes. Filesystem
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activity is never suspended during snapshot removal.
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The suspend time may be expanded by several minutes if a process
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is in the midst of removing many files as all the soft updates
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backlog must be cleared. Generally snapshots do not slow the system
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down appreciably except when removing many small files (i.e., any
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file less than 96Kb whose last block is a fragment) that are claimed
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by a snapshot. Here, the snapshot code must make a copy of every
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released fragment which slows the rate of file removal to about
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twenty files per second once the soft updates backlog limit is
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reached.
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How Snapshots Work
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For more general information on snapshots, please see:
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http://www.mckusick.com/softdep/
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