On Tue, Sep 08, 2015 at 08:12:15AM +0200, Christoph Hellwig wrote:

On Mon, Sep 07, 2015 at 05:28:55PM -0700, Shaohua Li wrote:

quoted

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.

Failing the I/O is ok too.
 

quoted

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.

I agree the benefit of FUA. In the system I'm testing, an Intel ssd
supports FUA, a sandisk SSD doesn't support FUA (this is the SSD we will
deploy for the log). This is AHCI with libata.fua=1. FUA isn't supported
by every SSD. If the log uses FUA by default, we will have a lot of FUA
write and performance is impacted.

I'll benchmark on a SSD from another vendor, which supports FUA, but FUA
write has poor performance in my last test.

Thanks,
Shaohua