[OmniOS-discuss] 4kn or 512e with ashift=12

[Chris Siebenmann]

> It should be noted that using a 512e disk as a 512n disk subjects
> you to a significant risk of silent corruption in the event of power
> loss. Because 512e disks does a read>modify>write operation to
> modify 512byte chunk of a 4k sector, zfs won't know about the other
> 7 corrupted 512e sectors in the event of a power loss during a write
> operation. So when discards the incomplete txg on reboot, it won't do
> anything about the other 7 512e sectors it doesn't know were affected.

 This is true; under normal circumstances you do not want to use a
512e drive in an ashift=9 vdev. However, if you have a dead 512n drive
and you have no remaining 512n spares, your choices are to run without
redundancy, to wedge in a 512e drive and accept the potential problems
on power failure (problems that can likely be fixed by scrubbing the
pool afterwards), or obtain enough additional drives (and perhaps
server(s)) to entirely rebuild the pool on 512e drives with ashift=12.

 In this situation, running with a 512e drive and accepting the
performance issues and potential exposure to power failures is basically
the lesser evil. (I wish ZFS was willing to accept this, but it isn't.)

	- cks