Fsck -n on mounted FS - how unreliable ?


 
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Operating Systems AIX Fsck -n on mounted FS - how unreliable ?
# 1  
Old 11-17-2014
Question Fsck -n on mounted FS - how unreliable ?

greetings all, we had SAN bobbles recently and so I ran fsck against all local FS on all systems.

our domino servers present a moving set of alleged corruption, both JFS and JFS2 FS.

Am I playing whack-a-mole with special effects (i.e. false positives) produced by vigorous filesystem activity while I'm trying to scan it ? Or is this potentially real enough so it's worth taking outages to unmount FS, etc. ?

TIA
# 2  
Old 11-17-2014
It never makes sense to fsck a mounted filesystem, especially a journalled one. The 'moving corruption' may be a race condition between the journal and the disk contents.

If you are truly concerned, scan them offline -- or at least remount them read-only.
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# 3  
Old 11-17-2014
Agreed.

Running fsck on a mounted filesystem is a worthless task. Unless you're lucky and it shows no errors - something I've never seen the few times I've witnessed an fsck of a mounted file system.

Because an indication of corruption isn't meaningful at all for a mounted file system.

If you're worried about corruption, you umount the filesystem and check it.
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# 4  
Old 11-18-2014
And be sure you have backuped those FS just in case : Have coruupted data is better than none...
Although its years since last time I faced FS issues I still remember the consequences...
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# 5  
Old 11-18-2014
thanks all for your replies. we took an outage, unmounted the filesys and sure enough, no errors. this was not a foreign concept to me, but last time we had a SAN prob I ran fsck on mounted FS and it turned out they did in fact have corruption when I unmounted them to fix them. Didn't want to chance it.

On that note, are any of you aware of a FS checking tool that works through the buffer cache so that the race with the disk contents on a mounted FS isn't there ? I'll unmount if I find I need to fix them, but to check them it would be nice to be able to do it live.
# 6  
Old 11-18-2014
That's rather contradictory, like asking for a bluer orange. If it were possible, it would end up being an even bigger hassle and risk than just unmounting it -- imagine losing 30 minutes of live changes on a corrupted and unrecoverable disk once you've realized "whoops, I really shouldn't have been writing to that". Mounting a bad filesystem read-write may have problems other than corruption -- it could work fine, for example, with one important file missing that you don't realize until later, after it's long past recoverable.

The closest you can do to what you want is mounting it read-only. Actually fixing it while it's mounted, read-only or not, is of course a recipe for a kernel panic.

I suppose it might also be possible at the disk level. Remove one mirror or something and scan it. You'll want to have it unmounted, or mounted read-only, while you do so. Again, though, you don't want to be writing changes to a disk that might be bad; you could lose your current changes.

Or some sort of scratch-disk union mount, so you're saving new changes in a temporary space until you've verified the disk is okay. Merging the two partitions would be difficult though.

I can't think of anything that's faster and less trouble than just doing the job properly in the first place. You can't re-bore your engine while it's running.

Last edited by Corona688; 11-18-2014 at 02:06 PM..
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# 7  
Old 11-18-2014
Code:
You can't re-bore your engine while it's running.

Classic!

We should have a hall of fame for comments like that!

;0)
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