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Summary: xfs_repair troubles

To: linux-xfs@xxxxxxxxxxx
Subject: Summary: xfs_repair troubles
From: Willi Langenberger <wlang@xxxxxxxxxxxxx>
Date: Wed, 22 May 2002 20:59:57 +0200
Reply-to: Willi.Langenberger@xxxxxxxxxxxxx
Sender: owner-linux-xfs@xxxxxxxxxxx
Hi!


Thanks to all who have helped -- finally I have my data back!!!!

Here is a short summary of our problems, and how we were able to solve
it ("/dev/sdb" was the block device of our partition).


Symptoms:

  1) "mount -t xfs /dev/sdb /mnt/home"

      runs for hours, working it's way over the disk....

  2) "xfs_repair -L /dev/sdb"

     version 2.0.3: 
       xfs_repair: xfs_log_recover.c:159: xlog_find_verify_log_record: 
Assertion `start_blk != 0 || *last_blk != start_blk' failed.
       Aborted (core dumped)

     version 2.0.3 with assertions disabled:
       same as 1): runs for hours

Reason:

  Log corruption that xfs_repair couldn't handle.

Cure:

  Use xfsprogs from CVS after May 15th 2002. This version handles
  log corruption more robust.

But our troubls continued. There was another...

Symptom:

  3) after a (appearently) successful run of xfs_repair (sample output
     http://slime.wu-wien.ac.at/xfs/repair.003.out) the filesystem was
     still corrupt; another run of xfs_repair reported the same
     corruptions...

Reason:

  The internal log overlaps with other filesystem data (no idea how
  this could happen!):

    # xfs_db -r /dev/sdb
    xfs_db: sb 0
    xfs_db: p
    magicnum = 0x58465342
    blocksize = 4096
    dblocks = 2000000
    rblocks = 0
    rextents = 0
    uuid = 2604c50c-3a11-4d67-a80c-156580239afb
    logstart = 4
    rootino = 128
    rbmino = 129
    rsumino = 130
    rextsize = 16
    agblocks = 1000
    agcount = 2000
    rbmblocks = 0
    logblocks = 512
    [...]

  ==> logstart=4, logblocks=512, rootino=128

    xfs_db: convert fsblock 4 daddr
    0x20 (32)

    xfs_db: convert ino 128 daddr
    0x40 (64)

  Thus: zeroing the log destroys the root inode. Later on, xfs_repair
  restores the root inode and corrupts the log...

Cure:

  Export the internal log to an external device. Steve Lord gave an
  explanation how to do this:

    http://marc.theaimsgroup.com/?l=linux-xfs&m=99535721116479&w=2

  However, i had to add a slight modification (because a recent
  xfs_repair is smarter and doesn't accept a change in the main
  superblock only):

    1. use xfsprogs >= 2.0 to avoid problems with endianess

    2. gather data from xfs_db (see above)
    blocksize = 4096
    logblocks = 512
    agcount = 2000

    3. select a partition to become the new log and zero it:

        dd if=/dev/zero of=/dev/NEWLOG bs=4096 count=512

    4. reset the log offset to zero (= external log) using xfs_db
    i did this for all agcount superblocks (setting only the main
    superblock doesn't do it, since xfs_repair compares all
    superblocks and takes the values from the majority):

      # echo >xfs_db.py <<EOF
#!/usr/bin/env python

AGCOUNT=2000                  # value from xfs_db
for i in xrange(AGCOUNT):     # loop through ags
    print "sb %d" % (i,)      # select ag superblock
    print "write logstart 0"  # set logstart to 0
print "quit"                  # exit xfs_db
EOF

      # ./xfs_db.py | xfs_db -x /dev/sdb >xfs_db.out


    5. run xfs_repair with the external log (that way no data on the
    filesystem are destroyed)

      # xfs_repair -L -l/dev/NEWLOG /dev/sdb

    6. mount the filesystem

      # mount -t xfs -r -o logdev=/dev/NEWLOG /dev/sdb /mnt/home


That way i was able to rescue all my data. Thanks again for your help
and for bringing this *great* filesystem to us!


\wlang{}

-- 
Willi.Langenberger@xxxxxxxxxxxxx                 Fax: +43/1/31336/702
Zentrum fuer Informatikdienste, Wirtschaftsuniversitaet Wien, Austria


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