Snapshots are never freed on at least 8.1 and 8.2

[Yamagi Burmeister]

Hello,
I'm not sure if this is a bug or the expected behavior but it seems quit
strange. On at least FreeBSD 8.1 and 8.2 UFS2 snapshots are never freed
while the filesystem is mounted. Therefor you have to remount every 20
snapshots which is quiet a pain when using "dump -L" or similar things
via cron.

Example:
--------

1. Create a new filesystem and copy some data on it:
   % mdmfs -s 512M md0 /mnt/
   % cp -r /usr/src/sys /mnt/

2. Create 20 snapshots (in tcsh-syntax):
   % foreach i ( 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 )
   % mksnap_ffs /mnt/ /mnt/.snap/${i}
   % end

3. "snapinfo -a" shows 20 snapshots.
   % root at saya:pts/3 ~> snapinfo -a
   /mnt/.snap/1
   /mnt/.snap/2
   /mnt/.snap/3
   /mnt/.snap/4
   /mnt/.snap/5
   /mnt/.snap/6
   /mnt/.snap/7
   /mnt/.snap/8
   /mnt/.snap/9
   /mnt/.snap/10
   /mnt/.snap/11
   /mnt/.snap/12
   /mnt/.snap/13
   /mnt/.snap/14
   /mnt/.snap/15
   /mnt/.snap/16
   /mnt/.snap/17
   /mnt/.snap/18
   /mnt/.snap/19
   /mnt/.snap/20

4. Further snapshots cannot be created since there's a limit of 20
    snapshots per filesystem:
   % mksnap_ffs /mnt/ /mnt/.snap/21
   mksnap_ffs: Cannot create snapshot /mnt/.snap/21: No space left on device

5. Now delete the snapshots:
   % rm -Rf /mnt/.snap/*

6. "snapinfo -a" tells us that there are no snapshots in the filesystem:
   % snapinfo -a
   %

7. But when we want create a new snapshot it fails:
   % mksnap_ffs /mnt/ /mnt/.snap/1
   mksnap_ffs: Cannot create snapshot /mnt/.snap/1: No space left on device

8. "ffsinfo /dev/md0 | grep snapinum" shows us 20 snapshots!
   % ffsinfo /dev/md0 | grep snapinum
   snapinum          int32_t[ 0]      0x00000004
   snapinum          int32_t[ 1]      0x00000005
   snapinum          int32_t[ 2]      0x00000006
   snapinum          int32_t[ 3]      0x00000007
   snapinum          int32_t[ 4]      0x00000008
   snapinum          int32_t[ 5]      0x00000009
   snapinum          int32_t[ 6]      0x0000000a
   snapinum          int32_t[ 7]      0x0000000b
   snapinum          int32_t[ 8]      0x0000000c
   snapinum          int32_t[ 9]      0x0000000d
   snapinum          int32_t[10]      0x0000000e
   snapinum          int32_t[11]      0x0000000f
   snapinum          int32_t[12]      0x00000010
   snapinum          int32_t[13]      0x00000011
   snapinum          int32_t[14]      0x00000012
   snapinum          int32_t[15]      0x00000013
   snapinum          int32_t[16]      0x00000014
   snapinum          int32_t[17]      0x00000015
   snapinum          int32_t[18]      0x00000016
   snapinum          int32_t[19]      0x00000017
   snapinum          int32_t[ 0]      0x00000000
   snapinum          int32_t[ 0]      0x00000000
   snapinum          int32_t[ 0]      0x00000000
   snapinum          int32_t[ 0]      0x00000000

9. Unmounting and remounting the filesystem let the kernel print some
    warnings. But afterwards (without fsck) the snapshots are gone and we
    can create new ones:
   % umount /mnt/
   % mount /dev/md0 /mnt/

   % dmesg | tail -n 20
   ffs_snapshot_mount: non-snapshot inode 4
   ffs_snapshot_mount: non-snapshot inode 5
   ffs_snapshot_mount: non-snapshot inode 6
   ffs_snapshot_mount: non-snapshot inode 7
   fs_snapshot_mount: non-snapshot inode 8
   ffs_snapshot_mount: non-snapshot inode 9
   ffs_snapshot_mount: non-snapshot inode 10
   ffs_snapshot_mount: non-snapshot inode 11
   ffs_snapshot_mount: non-snapshot inode 12
   ffs_snapshot_mount: non-snapshot inode 13
   ffs_snapshot_mount: non-snapshot inode 14
   fs_snapshot_mount: non-snapshot inode 15
   ffs_snapshot_mount: non-snapshot inode 16
   ffs_snapshot_mount: non-snapshot inode 17
   ffs_snapshot_mount: non-snapshot inode 18
   ffs_snapshot_mount: non-snapshot inode 19
   ffs_snapshot_mount: non-snapshot inode 20
   ffs_snapshot_mount: non-snapshot inode 21
   ffs_snapshot_mount: non-snapshot inode 22
   ffs_snapshot_mount: non-snapshot inode 23

   (This are the inodes shown by the ffsinfo above)

   % ffsinfo /dev/md0 | grep snapinum
   snapinum          int32_t[ 0]      0x00000000
   snapinum          int32_t[ 0]      0x00000000
   napinum          int32_t[ 0]      0x00000000
   snapinum          int32_t[ 0]      0x00000000
   snapinum          int32_t[ 0]      0x00000000

   % mksnap_ffs /mnt/ /mnt/.snap/1
   % snapinfo -a
   /mnt/.snap/1

Ciao,
Yamagi