linux-ext4 - Re: On-disk field assignments for metadata checksum and snapshots

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Message-ID: <20110915200512.GF28181@thunk.org>
Date:	Thu, 15 Sep 2011 16:05:12 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	djwong@...ibm.com, Amir Goldstein <amir73il@...il.com>,
	linux-ext4@...r.kernel.org
Subject: Re: On-disk field assignments for metadata checksum and snapshots

On Thu, Sep 15, 2011 at 01:10:41PM -0600, Andreas Dilger wrote:
> 
> This is also a possible alternative, though it makes for more fragments that
> need to be checksummed.  I think as a general rule it makes sense to store
> the checksum as the last word in the structure, if possible, so that the
> checksum can be computed in a single call.  This is already done for 128-byte
> inodes and for 32-byte group descriptors, but should also be done for the
> s_checksum field in the superblock (i.e. put it after s_reserved instead of
> before).

Or we just zero out the checksum field, checksum the entire data
structure, and then fill in the newly calculated checksum.  In fact
that's what I assumed Darrick was going to do.

	       	    	    	      - Ted
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