[OpenIndiana-discuss] Using large (3-4 TB) USB disks for backups

Andrew Gabriel illumos at cucumber.demon.co.uk
Wed Mar 26 18:35:20 UTC 2014

Have you got any other disk which can format as ashift=12?
(You could even use an iSCSI LUN from another system with blocksize set 
to 4k.)

If so, start by creating a zpool on that. Then attach your USB drive as 
a mirror, and it will have ashift=12. Then detach the original disk from 
the mirror, and expand the zpool to fill the whole drive.

Yes, we should have a command line option to set the ashift given how 
much trouble the failure to autodetect is causing in many cases (and you 
may want to override even a correct autodetect).

ashift is really a per-top level vdev property rather than a pool 
property, and we don't currently have any top level vdev properties 
handled by the zpool command.

However, for your usage case of just storing large send streams, I 
wouldn't go to any extra bother just to create ashift=12. There are 
other situations where it might actually make a noticeable difference, 
but I would be surprised if you see any in this case. You also reduce 
the scope for rescue if the pool gets damaged, as ashift=12 has only 32 
previous uberblocks available to work back through (versus 128 for 


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