zfsonlinux/zfs — pull request #959 by behlendorf (2012-09-12 15:45:16+00:00)
Title: Improve AF hard disk detection

Use the bdev_physical_block_size() interface to determine the
minimize write size which can be issued without incurring a
read-modify-write operation. This is used to set the ashift
correctly to prevent a performance penalty when using AF hard
disks.

Unfortunately, this interface isn't entirely reliable because
it's not uncommon for disks to misreport this value. For this
reason you may still need to manually set your ashift with:

zpool create -o ashift=12 ...

The solution to this in the upstream Illumos source was to add
a while list of known offending drives. Maintaining such a list
will be a burden, but it still may be worth doing if we can
detect a large number of these drives. This should be considered
as future work.

WARNING: This patch is not yet safe for inclusion in the master
branch since it may case issues for existing pools using AF
drives. See issue #955 or details. These issues must be
resolved before this change can be merged.

Reported-by: Richard Yao
Signed-off-by: Brian Behlendorf
Issue #916