On Mon, 2009-08-17 at 14:21 -0400, Greg Freemyer wrote: > On Mon, Aug 17, 2009 at 1:19 PM, James Bottomley<James.Bottomley@xxxxxxx> wrote: > > On Mon, 2009-08-17 at 13:08 -0400, Greg Freemyer wrote: > >> All, > >> > >> Seems like the high-level wrap-up of all this is: > >> > >> There are hopes that highly efficient SSDs will appear on the market > >> that can leverage a passthru non-coalescing discard feature. And that > >> a whitelist should be created to allow those SSDs to see discards > >> intermixed with the rest of the data i/o. > > > > That's not my conclusion. Mine was the NCQ drain would still be > > detremental to interleaved trim even if the drive could do it for zero > > cost. > > Maybe I misunderstood Jim Owens previous comment that designing for > devices that only meet the spec. was not his / Linus'es preference. > > Instead they want to have a whitelist enabled list of drives that > support trim / ncq without having to drain the queue. There's no way to do this. The spec explicitly requires that you not overlap tagged and untagged commands. The reason is fairly obvious: you wouldn't be able to separate the completions. > I just re-read his post and he did not explicitly say that, so maybe > I'm mis-representing it. > > >> For the other known cases: > >> > >> SSDs that meet the ata-8 spec, but don't exceed it > >> Enterprise SCSI > > > > No, SCSI will do WRITE_SAME/UNMAP as currently drafted in SBC3 > > > >> mdraid with SSD storage used to build raid5 / raid6 arrays > >> > >> Non-coalescing is believed detrimental, > > > > It is? Why? > > For the only compliant SSD in the wild, Mark has shown it to be true > via testing. He only said larger trims take longer. As I said previously, if it's a X+nY relationship, then we still benefit from accumulation up to some value of n. > For Enterprise SCSI, I thought you said a coalescing solution is > preferred. (I took that to mean non-coalescing is detremental. Not > true?). I'm trying to persuade the array vendors to speak for themselves, but it seems that UNMAP takes time. Of course, in SCSI, this is a taggable command so we don't have the drain overhead ... but then we can't do anything that would produce an undetermined state based on out of order tag execution either. > For mdraid, if the trims are not coalesced mdraid will have to either > ignore them, or coalesce them themselves. Having them come in bigger > discard ranges is clearly better. (ie. At least the size of a stripe, > so it can adjust the start / end sector to a stripe boundary.) If we did discard accumulation in-kernel (a big if), it would likely be at the request level; thus md and dm would automatically inherit it. dm/md are a problem for a userspace accumulation solution, though (although I suspect the request elevator can fix that). > >> but a regular flushing of the > >> unused blocks/sectors via a tool like Mark Lord has written should be > >> acceptable. > >> > >> Mark, I don't believe your tool really addresses the mdraid situation, > >> do you agree. ie. Since your bypassing most of the block stack, > >> mdraid has no way of snooping on / adjusting the discards you are > >> sending out. > >> > >> Thus the 2 solutions that have been worked on already seem to address > >> the needs of everything but mdraid. > > > > I count three: Mark Lord script via SG_IO. hch enhanced script via > > XFS_TRIM and willy current discard inline which he's considering > > coalescing for. > > I missed XFS_TRIM somehow. What benefit does XFS_TRIM provide at a > high level? Is it part of the realtime delete file process, or an > after the fact scanner? It guarantees that trim does not overlap allocations and writes on a running system, so it gives us safety of execution. James -- 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
