svn commit: r216230 - head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs

[Ivan Voras]

On 6 December 2010 21:18, John Baldwin <jhb at freebsd.org> wrote:
> On Monday, December 06, 2010 2:53:27 pm Pawel Jakub Dawidek wrote:
>> On Mon, Dec 06, 2010 at 08:35:36PM +0100, Ivan Voras wrote:
>> > Please persuade me on technical grounds why ashift, a property
>> > intended for address alignment, should not be set in this way. If your
>> > answer is "I don't know but you are still wrong because I say so" I
>> > will respect it and back it out but only until I/we discuss the
>> > question with upstream ZFS developers.
>>
>> No. You persuade me why changing ashift in ZFS, which, as the comment
>> clearly states is "device's minimum transfer size" is better and not
>> hackish than presenting the disk with properly configured sector size.
>> This can not only affect disks that still use 512 bytes sectors, but
>> doesn't fix the problem at all. It just works around the problem in ZFS
>> when configured on top of raw disks.
>>
>> What about other file systems? What about other GEOM classes? GELI is
>> great example here, as people use ZFS on top of GELI alot. GELI
>> integrity verification works in a way that not reporting disk sector
>> size properly will have huge negative performance impact. ZFS' ashift
>> won't change that.
>
> I am mostly on your side here, but I wonder if GELI shouldn't prefer the
> stripesize anyway?  For example, if you ran GELI on top of RAID-5 I imagine it
> would be far more performant for it to use stripe-size logical blocks instead
> of individual sectors for the underlying media.
>
> The RAID-5 argument also suggests that other filesystems should probably
> prefer stripe sizes to physical sector sizes when picking block sizes, etc.

For what it's worth, apparently linux has the concept of "physical"
and "logical" sector sizes (possibly in addition to "stripe size"),
with physical being 4096 and logical 512, for example:

# hdparm -I /dev/sde | grep size
Logical  Sector size:                   512 bytes
Physical Sector size:                  4096 bytes
device size with M = 1024*1024:     1430799 MBytes
device size with M = 1000*1000:     1500301 MBytes (1500 GB)