linux-kernel - Re: [RFD] Incremental fsck

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Message-ID: <20080109091656.GL3351@webber.adilger.int>
Date:	Wed, 9 Jan 2008 02:16:56 -0700
From:	Andreas Dilger <adilger@....com>
To:	Alan <alan@...eserver.org>
Cc:	Al Boldi <a1426z@...ab.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFD] Incremental fsck

Andi Kleen wrote:
>> Theodore Tso <tytso@....edu> writes:
>> > Now, there are good reasons for doing periodic checks every N mounts
>> > and after M months.  And it has to do with PC class hardware.  (Ted's
>> > aphorism: "PC class hardware is cr*p").
>>
>> If these reasons are good ones (some skepticism here) then the correct
>> way to really handle this would be to do regular background scrubbing
>> during runtime; ideally with metadata checksums so that you can actually
>> detect all corruption.
>>
>> But since fsck is so slow and disks are so big this whole thing
>> is a ticking time bomb now. e.g. it is not uncommon to require tens
>> of minutes or even hours of fsck time and some server that reboots
>> only every few months will eat that when it happens to reboot.
>> This means you get a quite long downtime.
>
> Has there been some thought about an incremental fsck?

While an _incremental_ fsck isn't so easy for existing filesystem types,
what is pretty easy to automate is making a read-only snapshot of a
filesystem via LVM/DM and then running e2fsck against that.  The kernel
and filesystem have hooks to flush the changes from cache and make the
on-disk state consistent.

You can then set the the ext[234] superblock mount count and last check
time via tune2fs if all is well, or schedule an outage if there are
inconsistencies found.

There is a copy of this script at:
http://osdir.com/ml/linux.lvm.devel/2003-04/msg00001.html

Note that it might need some tweaks to run with DM/LVM2 commands/output,
but is mostly what is needed.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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