Re: libata FUA revisited

Tejun Heo
2007-02-13T15:21:17Z

Hello, Robert.

Robert Hancock wrote:
[--snip--]
> On the NCQ side, I think it's pretty safe to assume that all controllers
> will handle it. Obviously I've verified it with sata_nv (at least that
> it doesn't blow up obviously), and the other two NCQ drivers we have,
> ahci and sata_sil24 just feed raw FIS data into the controller so there
> should be no issue with not supporting it.

FWIW, ICH6/7/8 ahci's clear PMP field when transmitting FIS. The reason
why I'm hesitant is because there is no way to tell whether the FUA bit
got honored or ignored. With extra opcode, it's okay because barrier
explicitly fails but if NCQ FUA is not supported, it will succeed
silently as normal write. Everything will be okay generally but the
barrier is done incorrectly and on a really bad day it will lead to
journal corruption.

So, actually, I was thinking about *always* using the non-NCQ FUA
opcode. As currently implemented, FUA request is always issued by
itself, so NCQ doesn't make any difference there. So, I think it would
be better to turn on FUA on driver-by-driver basis whether the
controller supports NCQ or not.

Well, I might be being too paranoid but silent FUA failure would be
really hard to diagnose if that ever happens (and I'm fairly certain
that it will on some firmwares).

--
tejun