On 2010-01-22, at 11:57, Ric Wheeler wrote:
On 01/22/2010 01:40 PM, Andreas Dilger wrote:
Reboot time:
(1) Try to mount the file system
(1) on mount failure, fsck the failed file system
Well, this is essentially what already happens with e2fsck today,
though
it correctly checks the filesystem for errors _first_, and _then_
mounts
the filesystem. Otherwise it isn't possible to fix the filesystem
after
mount, and mounting a filesystem with errors is a recipe for further
corruption and/or a crash/reboot cycle.
I think that we have to move towards an assumption that our
journalling code actually works - the goal should be that we can
*always* mount after a crash or clean reboot. That should be the
basic test case - pound on a file system, drop power to the storage
(and or server) and then on reboot, try to remount. Verification
would be in the QA test case to unmount and fsck to make sure our
journal was robust.
I think you are missing an important fact here. While e2fsck _always_
runs on a filesystem at boot time (or at least this is the recommended
configuration), this initial e2fsck run is only doing a very minimal
amount of work (i.e. it is NOT a full "e2fsck -f" run). It checks
that the superblock is sane, it recovers the journal, and it looks for
error flags written to the journal and/or superblock. If all of those
tests pass (i.e. less than a second of work) then the e2fsck run
passes (excluding periodic checking, which IMHO is the only issue
under discussion here).
Note that in a technique that I have used in the past (with
reiserfs) at large scale in actual deployments of hundreds of
thousands of file systems. It does work pretty well in practice.
The key here is that any fsck can be a huge delay, pretty much
unacceptable in production shops, where they might have multiple
file systems per box.
No, there is no delay if the filesystem does not have any errors. I
consider the lack of ANY minimal boot-time sanity checking a serious
problem with reiserfs and advised Hans many times to have minimal
sanity checks at boot.
The problem is that if the kernel (or a background snapshot e2fsck)
detects an error then the only way it can force a full check to
correct is to do this on the next boot, by storing some information in
the superblock. If the filesystem is mounted at boot time without
even a minimal check for such error flags in the superblock then the
error may never be corrected, and in fact may cause cascading
corruption elsewhere in the filesystem (e.g. corrupt bitmaps, bad
indirect block pointers, etc).
If the filesystem is mounted with "errors=panic" (often done with HA
configs to allow failover in case of node/cable/HBA errors) then any
corruption on disk will just result in the node getting stuck in a
reboot cycle with no automated e2fsck being run.
While up and running, do a periodic check with the snapshot trick.
Yes, this is intended to reset the periodic mount/time counter to
avoid
the non-error boot-time check. If that is not running correctly
then the
periodic check would still be done as a fail-safe measure.
Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.
Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.
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