Re: Accelerating non-standard disk types

[Stefan Hajnoczi]

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.

Stefan

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