linux-ext4 - Re: Some thoughts about providing data block checksumming for ext4

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Message-ID: <20141127000722.GA310@redhat.com>
Date:	Wed, 26 Nov 2014 19:07:22 -0500
From:	Mike Snitzer <snitzer@...hat.com>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	"Theodore Ts'o" <tytso@....edu>, dm-devel@...hat.com,
	linux-ext4@...r.kernel.org
Subject: Re: Some thoughts about providing data block checksumming for ext4

On Wed, Nov 26 2014 at  6:47pm -0500,
Darrick J. Wong <darrick.wong@...cle.com> wrote:

> Sigh...
> 
> Well, I wrote up a preliminary version of dm-checksum and then
> realized that I've pretty much just built a crappier version of
> dm-dedupe, but without the dedupe part.  Given that it stores
> checksums in a btree which claims to be robust through failures and
> gives us automatic deduplication, I wonder if it we could achieve our
> aims by modifying dm-dedupe to verify the checksums on the read path?
> 
> I guess it would be interesting to see how bad the performance hit is
> with the online dedupe part enabled or disabled.  dm-dedupe v2 went
> out on the mailing list last August, which I missed. :(
> 
> Unless... there's a specific reason nobody mentioned dm-dedupe here?

As you may have seen in the dm-dedup thread, we need to actively
review/test that target (if your initial review focus is on extending it
to _optionally_ verify the checksums on the read path then so be it).

See: https://www.redhat.com/archives/dm-devel/2014-November/msg00114.html
Specifically, the git branch that builds on v2 based on my initial
review of v2:

git://git.fsl.cs.stonybrook.edu/scm/git/linux-dmdedup
branch: dm-dedup-devel

Your help on getting dm-dedup upstream would be very much appreciated.

Thanks,
Mike
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