Updated ashift optimization patch

[Steven Hartland]

Few things I've noticed:-

1. in vdev_geom.c is there a reason you use:
+ *physical_ashift = 0;
+ if (pp->stripesize)
+  *physical_ashift = highbit(pp->stripesize) - 1;

Instead of:
+ *physical_ashift = highbit(MAX(pp->stripesize, SPA_MINBLOCKSIZE)) - 1;

2. sysctl_vfs_zfs_max_auto_ashift should also limit the min to >
   SPA_MINBLOCKSHIFT

3. sysctl_vfs_zfs_max_auto_ashift should error if the value is out
   of range instead of clamping it.

One thing we did here, which would be good to see in this patch, is to add
an overall min system ashift as thie enables admins to configure pools
to be compatible with future disks they are likely to use e.g. min ashift
12 (4k compatible). This could be left at 9 by default for max compatibility
but personally I'd suggest 12.

    Regard
    Steve

----- Original Message ----- 
From: "Justin T. Gibbs" <gibbs at FreeBSD.org>
To: <zfs-devel at freebsd.org>; <zfs at lists.illumos.org>
Cc: "Richard Yao" <ryao at gentoo.org>
Sent: Monday, August 19, 2013 12:42 AM
Subject: Updated ashift optimization patch


Today I re-merged all of Spectra's ashift related changes into FreeBSD/head.  Here's the current state of the patch.  See the top 
of the patch for proposed commit message/justification.  The majority of it is ZFS platform agnostic.

Later this week I'll get a stock FreeBSD/head system running and run the test suites in order to verify that I haven't botched 
anything in the merge.  I'll also see about merging it to one of Will's Illumos VMs so I can generate a webrev.  In the mean time, 
comments, concerns, improvements welcome.

--
Justin




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