linux-ext4 - Re: [dm-devel] Some thoughts about providing data block checksumming for ext4

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Message-ID: <alpine.LRH.2.02.1411041703590.7145@file01.intranet.prod.int.rdu2.redhat.com>
Date:	Tue, 4 Nov 2014 17:06:30 -0500 (EST)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	"Theodore Ts'o" <tytso@....edu>
cc:	linux-ext4@...r.kernel.org, dm-devel@...hat.com
Subject: Re: [dm-devel] Some thoughts about providing data block checksumming
 for ext4



> > Recovery after a power fail
> > ---------------------------
> > 
> > If the dm-protected device was not cleanly shut down, then we need to
> > examine all of the checksum blocks in the Active Area.  For each
> > checksum block in the AA, the checksums for all of their data blocks
> > should machine either the checksum found in the AA, or the checksum
> > found in the checksum block in the checksum group.
> 
> ... and if the checksum of the block matches BOTH the checksum in the AA 
> and the checksum in the checksum group (because of checksum function 
> collision), you don't know which 4-bit nibble belongs to the data in the 
> block.

Though, I realize that you could avoid this problem by selecting the 
appropriate checksum function - that never results in collision if the 
4-bit nibble differs.

Mikulas
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