Re: raid5-cache I/O path improvements

[Christoph Hellwig <hch@lst.de>]

On Mon, Sep 07, 2015 at 05:28:55PM -0700, Shaohua Li wrote:
> Hi Christoph,
> Thanks for these work. Yes, I/O error handling is in the plan. We could
> simplify panic (people here like this option) or report error and bypass
> log. Any way an option is good.

I think the sensible thing in general is to fail the I/O.  Once we have
a cache devie the assumption is that a) write holes are properly handled,
and we b) do all kinds of optimizations based on the presensce of the
log device like not passing through flush requests or skippign resync.

Having the cache device suddenly disappear will alwasy break a) and
require a lot of hairy code only used in failure cases to undo the
rest.

> For the patches, FUA write does simplify things a lot. However, I tried
> it before, the performance is quite bad in SSD. FUA is off in SATA by
> default, the emulation is farily slow because FLUSH request isn't NCQ
> command. I tried to enable FUA in SATA too, FUA write is still slow in
> the SSD I tested. Other than this one, other patches look good:

Pretty much every SSD (and modern disk driver) supports FUA.  Please
benchmark with libata.fua=Y, as I think the simplifcation is absolutely
worth it.  On my SSDs using it gives far lower latency for writes,
nevermind nvmdimm where it's also essential as the flush statemchine
increases the write latency by an order of magnitude.

Tejun, do you have any updates on libata vs FUA?  We onable it
by default for a while in 2012, but then Jeff reverted it with a rather
non-descriptive commit message.

Also NVMe or SAS SSDs will benefit heavily from the FUA bit.