Message-ID: <424A2BD0.5010609@comcast.net>
Date: Tue, 29 Mar 2005 23:32:16 -0500
From: John Richard Moser <nigelenki@comcast.net>
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To: linux-kernel@vger.kernel.org
Subject: Aligning file system data
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How likely is it that I can actually align stuff to 31.5KiB on the
physical disk, i.e. have each block be a track?

Rather than leveraging the track cache, would it be less expensive for
me to simply read in blocks totaling about 16 or 32KiB all at once?


Let's say I have two situations...

A)
  My blocks are all 31.5KiB (512 bytes/sector * 63 sectors) and aligned
to tracks.  The track cache on the disk stores the entire block, so
repeted reads to the disk are 0mS seek.  I leverage this to read a
couple sectors at a time and seek as I care within the block while it's
cached, making several requests to the ATA device.

B)
  My blocks are all 32KiB and cross track boundaries.  All of them exist
in part in two separate tracks.  Upon reading a block, I request the
entire block and work with it in main memory.

Which situation has less overhead?

C)
  My blocks are all 31.5KiB and perfectly aligned within tracks.  I read
the entire block as in (B) and work with it in main memory.

How much more latency is involved in (B) than in (C)?  Does crossing a
track boundary incur anything expensive?


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