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: > > 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. > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
