On Sun, Aug 30, 2009 at 06:48:29PM -0400, Christoph Hellwig wrote: > As I've recently worked on all sides of the discard battle (filesystem > support, initiator support, and target support) here are my notes: > > - WRITE_SAME is extremly nice to implement for both the initiator and > target. It has the LBA and len exactly in the same place as normal > 16 byte commands, the payload length is fixed to one block, which > we can allocate once and zero so that we don't even need any memory > allocations for this command in the initiator. > - UNMAP is a pain to implement in both initiator and target. Not > actuall having the LBA/len information in the cdb but in the payload > is at least a minor incovenience in the initator, and quite annoying > in the target as we now need to process payload data in the fastpath, > which we otherwise only do for slow path CDBs. This will be > especially bad for split kernel/user target implementations. > > Now the weird design of UNMAP of course has a rather (besides some > apparent pissing contest at NetApp about who can't come with the worst > possible protocol specifications, whose results can be seen in NFSv4 > and iSer), and that is that it allows dicarding of multiple > discontinguous ranges. This sentence no object ;-) > Doing so is really bad for the filesystem as > it requires it to track multiple outstanding discard requests, which > requires locking, and book keeping to make sure we do not re-use these > blocks before they are discarded. Yeah, but we need to do that for TRIM anyway. While we're doing it for TRIM, we might as well do it for UNMAP. > And at least for my target design it does not provide any measureable > benefits at all, the discard operations are mapped to a hole punch > ioctl on a filesystem, which has a constant basic overhead for each > region punched (synchronous transaction commit) and a small linear > cost per extent removed. The only benefit of the multiple rangs unmap > would be a saving of protocol roundtrips. Sure, but you've got a relatively sane underpinning. NAND is pretty insane, and the aggregation can actually go a long way to helping with some of the problems. > Now that is interestingly actually a downside at least for my still > rather dumb target implementation with a typical Linux filesystem > workload on the initiator side. If we actually do a lot different unmap > operations in a single unmap command it can start to take significant > amounts of time, and do to Linux waiting for queue drains frequently > due to the barrier implementations we will end up waiting for the unmap > command. OK, but that's because you've implemented a single-range ioctl. If we had an ioctl which let you discard multiple ranges, it would actually be faster (due to the barriers) than implementing a WRITE SAME. -- Matthew Wilcox Intel Open Source Technology Centre "Bill, look, we understand that you're interested in selling us this operating system, but compare it to ours. We can't possibly take such a retrograde step." -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
