effect of differing spindle speeds on prospective zfs vdevs

[tech-lists]

Hi,

On Sat, Dec 05, 2020 at 08:51:08AM -0500, Paul Mather wrote:
> IIRC, ZFS pools have a single ashift for the entire pool, so you should 
> set it to accommodate the 4096/4096 devices to avoid performance 
> degradation.  I believe it defaults to that now, and should auto-detect 
> anyway.  But, in a mixed setup of vdevs like you have, you should be 
> using ashift=12.
>
> I believe having an ashift=9 on your mixed-drive setup would have the 
> biggest performance impact in terms of reducing performance.

Part of my confusion about the ashift thing is I thought ashift=9 was for
512/512 logical/physical. Is this still the case?

On a different machine which has been running since FreeBSD12 was -current,
one of the disks in the array went bang. zdb shows ashift=9 (as was default
when it was created). The only available replacement was an otherwise 
identical disk but 512 logical/4096 physical. zpool status mildly warns 
about preformance degradation like this:

ada2    ONLINE       0     0     0  block size: 512B configured, 4096B native

  state: ONLINE
status: One or more devices are configured to use a non-native block size.
      Expect reduced performance.
action: Replace affected devices with devices that support the
      configured block size, or migrate data to a properly configured
      pool.

The other part of my confusion is that I understood zfs to set its own 
blocksize on the fly.

(I guess there must be some performance degradation but it's not
yet enough for me to notice. Or it might only be noticable if low on space).
-- 
J.
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