Date:	Fri, 22 Jan 2010 15:58:45 -0500
From:	Valerie Aurora <vaurora@...hat.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Andreas Dilger <adilger@....com>,
	Ric Wheeler <rwheeler@...hat.com>, tytso@....edu,
	ext4 development <linux-ext4@...r.kernel.org>,
	Bill Nottingham <notting@...hat.com>
Subject: Re: [PATCH] default max mount count to unused

On Fri, Jan 22, 2010 at 01:59:26PM -0600, Eric Sandeen wrote:
> Andreas Dilger wrote:
> > 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
> 
> well, there is a delay if it's the magical Nth time or the magical Nth
> hour, right?  Which is what we're trying to avoid.
> 
> > 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.
> 
> I have no problem with checking an fs marked with errors...

Yes, I think we are all in violent agreement on this.

> > 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).
> 
> Mmmhm, so if we mark it with the error and a next boot fscks... I can
> live with that.
> 
> I just want to avoid the "we scheduled a brief window to upgrade the kernel,
> and the next time we booted we got a 3-hour fsck that we didn't expect,
> and we were afraid to stop it, but oh well it was clean anyway" scenario.
> 
> I guess the higher-level discussion to have is
> 
> a) what are the errors and the root-causes that the forced periodic
>    checks are intended to catch
> 
> and
> 
> b) what are the pros and cons of periodic checking for those errors,
>    vs catching them at runtime and scheduling a fsck as a result.
> 
> or maybe it's "how much of a nanny-state do we want to be?" :)

Do any other file systems have this "fsck on N reboots/N days up"
behavior?  Is ext3/ext4 the odd one out?

-VAL
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