FreeBSD/Linux Kernel Cross Reference
sys/ufs/ffs/README.snapshot

     $FreeBSD: releng/8.0/sys/ufs/ffs/README.snapshot 107762 2002-12-12 00:31:45Z trhodes $
     
     Soft Updates Status
     
     As is detailed in the operational information below, snapshots
     are definitely alpha-test code and are NOT yet ready for production
     use. Much remains to be done to make them really useful, but I
     wanted to let folks get a chance to try it out and start reporting
     bugs and other shortcomings. Such reports should be sent to
    Kirk McKusick <mckusick@mckusick.com>.
    
    
    Snapshot Copyright Restrictions
    
    Snapshots have been introduced to FreeBSD with a `Berkeley-style'
    copyright. The file implementing snapshots resides in the sys/ufs/ffs
    directory and is compiled into the generic kernel by default.
    
    
    Using Snapshots
    
    To create a snapshot of your /var filesystem, run the command:
    
            mount -u -o snapshot /var/snapshot/snap1 /var
    
    This command will take a snapshot of your /var filesystem and
    leave it in the file /var/snapshot/snap1. Note that snapshot
    files must be created in the filesystem that is being snapshotted.
    I use the convention of putting a `snapshot' directory at the
    root of each filesystem into which I can place snapshots.
    You may create up to 20 snapshots per filesystem. Active snapshots
    are recorded in the superblock, so they persist across unmount
    and remount operations and across system reboots. When you
    are done with a snapshot, it can be removed with the `rm'
    command. Snapshots may be removed in any order, however you
    may not get back all the space contained in the snapshot as
    another snapshot may claim some of the blocks that it is releasing. 
    Note that the `schg' flag is set on snapshots to ensure that
    not even the root user can write to them. The unlink command
    makes an exception for snapshot files in that it allows them
    to be removed even though they have the `schg' flag set, so it
    is not necessary to clear the `schg' flag before removing a
    snapshot file.
    
    Once you have taken a snapshot, there are three interesting
    things that you can do with it:
    
    1) Run fsck on the snapshot file. Assuming that the filesystem
       was clean when it was mounted, you should always get a clean
       (and unchanging) result from running fsck on the snapshot.
       If you are running with soft updates and rebooted after a
       crash without cleaning up the filesystem, then fsck of the
       snapshot may find missing blocks and inodes or inodes with
       link counts that are too high. I have not yet added the
       system calls to allow fsck to add these missing resources
       back to the filesystem - that will be added once the basic
       snapshot code is working properly. So, view those reports
       as informational for now.
    
    2) Run dump on the snapshot. You will get a dump that is
       consistent with the filesystem as of the timestamp of the
       snapshot.
    
    3) Mount the snapshot as a frozen image of the filesystem.
       To mount the snapshot /var/snapshot/snap1:
    
            mdconfig -a -t vnode -f /var/snapshot/snap1 -u 4
            mount -r /dev/md4 /mnt
    
       You can now cruise around your frozen /var filesystem
       at /mnt. Everything will be in the same state that it
       was at the time the snapshot was taken. The one exception
       is that any earlier snapshots will appear as zero length
       files. When you are done with the mounted snapshot:
    
            umount /mnt
            mdconfig -d -u 4
    
       Note that under some circumstances, the process accessing
       the frozen filesystem may deadlock. I am aware of this
       problem, but the solution is not simple. It requires
       using buffer read locks rather than exclusive locks when
       traversing the inode indirect blocks. Until this problem
       is fixed, you should avoid putting mounted snapshots into
       production.
    
    
    Performance
    
    It takes about 30 seconds to create a snapshot of an 8Gb filesystem.
    Of that time 25 seconds is spent in preparation; filesystem activity
    is only suspended for the final 5 seconds of that period. Snapshot
    removal of an 8Gb filesystem takes about two minutes. Filesystem
    activity is never suspended during snapshot removal.
    
    The suspend time may be expanded by several minutes if a process
    is in the midst of removing many files as all the soft updates
    backlog must be cleared. Generally snapshots do not slow the system
    down appreciably except when removing many small files (i.e., any
   file less than 96Kb whose last block is a fragment) that are claimed
   by a snapshot. Here, the snapshot code must make a copy of every
   released fragment which slows the rate of file removal to about
   twenty files per second once the soft updates backlog limit is
   reached.
   
   
   How Snapshots Work
   
   For more general information on snapshots, please see:
           http://www.mckusick.com/softdep/

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