Re: Question about file system checks

In the last episode (Mar 28), Ivan Voras said:
Danny Pansters wrote:
Generally I can say that with freebsd even if you pull the plug and
then let it reboot and do the automatical background fsck you'll
likely loose only that one file you might have been editing while
(or just before) you unplugged the box.

Stress testing I've done suggests otherwise :) I've literally
repeatedly pulled the plug of a server in a controlled environment,
and with a network logging of (a high load of) file system
operations. My results show that UFS+SU and ZFS on FreeBSD loose *the
most* files (and in case of UFS+SU especially directories), than any
of: jfs, xfs, reiser3 (on Linux 2.6.22) and NTFS (on Windows 2003
Server). ext3 is somewhat similar to UFS+SU, though about 30% better
at not loosing files.

Note that you can tweak the SU caching time by adjusting the sysctls
kern.{meta,dir,file}delay. Take them down to 10 seconds instead of 30
and you'll lose less files (at the cost of more disk I/O of course).

Some other notes from this proceeding:

1. UFS+gjournal looses the least, but it's also the slowest.
2. UFS+SU had no truncated files or files of unexpected length (apparently
it just looses the file that would end up in this state)
3. XFS and JFS end up with a *huge* number of files that are truncated or
of unexpected length (40%-50%!)
4. In no case has any of the above file systems gone completely corrupted
or lost any of the files/directories not being updated.
5. ZFS on FreeBSD was the fastest, in the sense of creating the most files
during this benchmark (though speed was not the target for this benchmark
so this is a low-quality observation), closely followed by JFS and XFS.

ZFS's transaction commit interval is only 5 seconds (see txg_time in
uts/common/fs/zfs/txg.c); how many more files/second did it create vs
the others to be able to lose the most files in that window? :)

6. ZFS crashed the kernel at least once.

--
Dan Nelson
dnelson@xxxxxxxxxxxxxxx
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Relevant Pages

  • Re: Question about file system checks
    ... then let it reboot and do the automatical background fsck you'll likely loose only that one file you might have been editing while you unplugged the box. ... My results show that UFS+SU and ZFS on FreeBSD loose *the most* files, than any of: jfs, xfs, reiser3 (on ... UFS+SU had no truncated files or files of unexpected length ...
    (freebsd-stable)
  • Re: Question about file system checks
    ... it reboot and do the automatical background fsck you'll likely loose only that one file you might have been editing while you unplugged the box. ... My results show that UFS+SU and ZFS on FreeBSD loose *the most* files, than any of: jfs, xfs, reiser3 (on ... ZFS on FreeBSD was the fastest, in the sense of creating the most files during this benchmark, closely followed by JFS and XFS. ...
    (freebsd-stable)