[Grml] Write barriers for journaling filesystems

Mark 27e3kk302 at sneakemail.com
Sun Jan 21 04:32:26 CET 2007

Testing exercises with XFS, ext3, and power failure.  I found advice to
use write barriers.  It appears that any journaling file system
(XFS/Reiser/ext3) will benefit w.r.t. power failures.  Almost all hard
drives, IDE, SATA, USB, SCSI have a ~multimegabyte write cache nowadays.

     "OK, enough theory. Do this: mount all of your filesystems with I/O
     barrier turned on. Something like this (example line from my
     /etc/fstab):  /dev/sda5 / ext3 defaults,barrier=1 1 1"  

     "Fortunately, you can have your cake and eat it too. The trick is
     to implement IO barriers using the CACHE FLUSH and/or FUA commands.
     That way you can have the performance and MTBF benefits of
     write-back caching while still having a safe fsync() (safe as in
     doesn't return before data is on the platters)."  

It seems like maybe a good default setting for journaled filesystems,
especially on USB (which users often yank out...).


P.S. Update on ZFS.  Apple will include it in their next OS release
(mika: not as default/boot, still HFS+ for that).  Also, Sun may release
ZFS under GPLv3 "eventually."

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