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Date: Mon, 5 Dec 2011 11:40:15 -0500
From: "Ted Ts'o" <tytso@mit.edu>
To: "Darrick J. Wong" <djwong@us.ibm.com>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>,
        Sunil Mushran <sunil.mushran@oracle.com>,
        Martin K Petersen <martin.petersen@oracle.com>,
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        Joel Becker <jlbec@evilplan.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-ext4@vger.kernel.org, Coly Li <colyli@gmail.com>
Subject: Re: [PATCH 10/22] ext4: Verify and calculate checksums for extent
 tree blocks
Message-ID: <20111205164015.GD32031@thunk.org>
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On Mon, Nov 28, 2011 at 03:27:25PM -0800, Darrick J. Wong wrote:
>  /*
> + * This is the extent tail on-disk structure.
> + * All other extent structures are 12 bytes long.  It turns out that
> + * block_size % 12 >= 4 for all valid block sizes (1k, 2k, 4k).

More generally, block_size % 12 is >= 4 for all powers of 2 greater
than 12 bytes.  I'd probably remove (1k, 2k, 4k) because that might
scare people who might say, "hey!  I'm using a 16k block size on my
HPC system running on a Power or Itanium box --- am I going to be OK?".
Valid block sizes go up to the page size of the hardware in question...

      	    	     	       	    	    - Ted