On Wed, 15 Feb 2012, Bill Sommerfeld wrote:
last fall I did a little experimentation with a pair of WD20EARS drives (which
are 2TB "advanced format" drives with 4K physical and 512 byte logical sectors).
In my admittedly not particularly demanding and not particularly scientific
tests, I saw little difference in performance between ashift=9 (512) and
ashift=12 (4K) but significantly increased disk space usage, so I left them as
ashift=9.
The performance difference is primarily while writing because the
drive may need to read 4K from the media so that it can update 512
bytes in it. With the default zfs filesystem blocksize of 128K and
due to zfs COW overwriting large numbers of contiguous disk blocks,
this need to read before update is considerably diminished. Zfs
metadata would be the primary problem. If zfs filesystem blocks are
not 4k aligned, then there are still additional media reads at the
start and end of the written zfs block. 4k alignment of filesystem
data blocks is clearly important.
Has anyone taken a more thorough look at this to better characterize the space
vs. speed tradeoff?
There has been some discussion of this on the FreeBSD filesystem list.
Bob
--
Bob Friesenhahn
[email protected], http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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