linux-ext4 - Re: [PATCH 02/22] ext4: Create a rocompat flag for extended metadata checksumming

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Message-ID: <20111205192450.GA7137@tux1.beaverton.ibm.com>
Date:	Mon, 5 Dec 2011 11:24:50 -0800
From:	"Darrick J. Wong" <djwong@...ibm.com>
To:	"Ted Ts'o" <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 02/22] ext4: Create a rocompat flag for extended
	metadata checksumming

On Mon, Dec 05, 2011 at 10:41:33AM -0500, Ted Ts'o wrote:
> (Reviews sent to ext4 list only)
> 
> On Mon, Nov 28, 2011 at 03:26:29PM -0800, Darrick J. Wong wrote:
> > This patch introduces a rocompat feature flag to signal the presence
> > of checksums for metadata blocks.  It also provides storage for a
> > precomputed UUID checksum.
> > 
> > +
> > +	/* Precomputed FS UUID checksum */
> > +	__u32 s_uuid_csum;
> 
> This is really a checksum seed, right?  Calling it a UUID is going to
> be confusing because UUID's are expected to be 128 bits, and something
> which is 32 bits can't possibly be a universally unique.

It's the checksum of the FS UUID, so I guess 'seed' describes it.  I wasn't
trying to call it a UUID.

I also need to clarify the changelog, apparently.  s_uuid_csum is *in-core*
storage for a precomputed UUID checksum.  This piece (and i_uuid_inum_csum) are
computed at load time, just in case the user changed the UUID when the fs was
offline.

> I'd suggest calling this "s_csum_seed", and we explain it as a
> random seed which is selected when the file system is formatted.

Not necessarily random, since you can mkfs.ext4 -U.

> The commit description could also explain that this is used so we can
> detect old metadata blocks left over from a previous use of the
> storage device.

Ok.

--D

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