[zfs-discuss] ashift=12 on 512 bytes disks?

Gionatan Danti g.danti at assyoma.it
Sun Dec 17 04:05:06 EST 2017

Hi all,
I recently created a new ZFS pool using 4x WD Gold 2TB (configured in 
mirrored pairs) and 2x Samsung 850 EVO 500GB for caching ans SLOG.

The main pool disks (WD Gold) have 512-bytes sectors and I left ashift 
to the default value (ashift=9). Cache and SLOG were added with "-o 
ashift=12" to align at 4K boundaries.

Now I wonder if the original choice to use ashift=9 on the main disks 
was the better one, or if it can bite me in the future. Suppose a disk 
fails and I can not replace it with another 512-bytes disk, rather a 4K 
HD must be used. I know I can manually set the correct ashift value (12) 
during replacing; however, I wonder if this "mixed" mode (ashift=9 on 
some disks, ashift=12 on the new 4K one) can lead to lower performance 
and/or other problems.

The obvious solution would be to recreate the entire pool with 
ashift=12, treating all disks as 4K ones. I know this implies some space 
loss, but it should be no too severe.

Any suggestions?

Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti at assyoma.it - info at assyoma.it
GPG public key ID: FF5F32A8

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