On Tue, May 31, 2022 at 03:06:20AM +0000, Raphael Norwitz wrote: > On Wed, May 25, 2022 at 05:00:04PM +0100, Stefan Hajnoczi wrote: > > On Thu, May 19, 2022 at 06:39:39PM +0000, Raphael Norwitz wrote: > > > On Tue, May 17, 2022 at 03:53:52PM +0200, Paolo Bonzini wrote: > > > > On 5/16/22 19:38, Raphael Norwitz wrote: > > > > > [1] Keep using the SCSI translation in QEMU but back vDisks with a > > > > > vhost-user-scsi or vhost-user-blk backend device. > > > > > [2] Implement SATA and IDE emulation with vfio-user (likely with an > > > > > SPDK > > > > > client?). > > > > > [3] We've also been looking at your libblkio library. From your > > > > > description in > > > > > https://urldefense.proofpoint.com/v2/url?u=https-3A__lists.gnu.org_archive_html_qemu-2Ddevel_2021-2D04_msg06146.html&d=DwICaQ&c=s883GpUCOChKOHiocYtGcg&r=In4gmR1pGzKB8G5p6LUrWqkSMec2L5EtXZow_FZNJZk&m=wBSqcw0cal3wPP87YIKgFgmqMHjGCC3apYf4wCn1SIrX6GW_FR-J9wO68v-cyrpn&s=CP-6ZY-gqgQ2zLAJdR8WVTrMBoqmFHilGvW_qnf2myU&e= > > > > > it > > > > > sounds like it may definitely play a role here, and possibly provide > > > > > the > > > > > nessesary abstractions to back I/O from these emulated disks to any > > > > > backends we may want? > > > > > > > > First of all: have you benchmarked it? How much time is spent on MMIO > > > > vs. > > > > disk I/O? > > > > > > > > > > Good point - we haven’t benchmarked the emulation, exit and translation > > > overheads - it is very possible speeding up disk I/O may not have a huge > > > impact. We would definitely benchmark this before exploring any of the > > > options seriously, but as you rightly note, performance is not the only > > > motivation here. > > > > > > > Of the options above, the most interesting to me is to implement a > > > > vhost-user-blk/vhost-user-scsi backend in QEMU, similar to the NVMe one, > > > > that would translate I/O submissions to virtqueue (including polling > > > > and the > > > > like) and could be used with SATA. > > > > > > > > > > We were certainly eyeing [1] as the most viable in the immediate future. > > > That said, since a vhost-user-blk driver has been added to libblkio, [3] > > > also sounds like a strong option. Do you see any long term benefit to > > > translating SATA/IDE submissions to virtqueues in a world where libblkio > > > is to be adopted? > > > > > > > For IDE specifically, I'm not sure how much it can be sped up since it > > > > has > > > > only 1 in-flight operation. I think using KVM coalesced I/O could > > > > provide > > > > an interesting boost (assuming instant or near-instant reply from the > > > > backend). If all you're interested in however is not really > > > > performance, > > > > but rather having a single "connection" to your back end, vhost-user is > > > > certainly an option. > > > > > > > > > > Interesting - I will take a look at KVM coalesced I/O. > > > > > > You’re totally right though, performance is not our main interest for > > > these disk types. I should have emphasized offload rather than > > > acceleration and performance. We would prefer to QA and support as few > > > data paths as possible, and a vhost-user offload mechanism would allow > > > us to use the same path for all I/O. I imagine other QEMU users who > > > offload to backends like SPDK and use SATA/IDE disk types may feel > > > similarly? > > > > It's nice to have a single target (e.g. vhost-user-blk in SPDK) that > > handles all disk I/O. On the other hand, QEMU would still have the > > IDE/SATA emulation and libblkio vhost-user-blk driver, so in the end it > > may not reduce the amount of code that you need to support. > > > > Apologies for the late reply - I was on PTO. > > For us it’s not so much about the overall LOC we support. We have our > own iSCSI client implementation with embedded business logic which we > use for SCSI disks. Continuing to support SATA and IDE disks without our > implementation has been really troublesome so, even if it means more > LOC, we would really like to unify our data path at least at the iSCSI > layer. > > While the overall code may not be reduced so much for many others today, > it may make a significant difference in the future. I can imagine some > QEMU users may want to deprecate (or not implement) iSCSI target support > in favor of NVMe over fabrics and still support these disk types. Being > able to offload the transport layer via vhost-user-blk (either with some > added logic on top of the existing SCSI translation layer or with > libblkio) would make this easy. > > Does that sound reasonable?
Yes. Stefan
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