Re: Accelerating non-standard disk types

[Stefan Hajnoczi]

On Mon, May 16, 2022 at 05:38:31PM +0000, Raphael Norwitz wrote:
> Hey Stefan,
> 
> We've been thinking about ways to accelerate other disk types such as
> SATA and IDE rather than translating to SCSI and using QEMU's iSCSI
> driver, with existing and more performant backends such as SPDK. We
> think there are some options worth exploring:
> 
> [1] Keep using the SCSI translation in QEMU but back vDisks with a
> vhost-user-scsi or vhost-user-blk backend device.

If I understand correctly the idea is to have a QEMU Block Driver that
connects to SPDK using vhost-user-scsi/blk?

> [2] Implement SATA and IDE emulation with vfio-user (likely with an SPDK
> client?).

This is definitely the option with the lowest overhead. I'm not sure if
implementing SATA and IDE emulation in SPDK is worth the effort for
saving the last few cycles.

> [3] We've also been looking at your libblkio library. From your
> description in
> https://lists.gnu.org/archive/html/qemu-devel/2021-04/msg06146.html 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?

Kevin Wolf has contributed a vhost-user-blk driver for libblkio. This
lets you achieve #1 using QEMU's libblkio Block Driver. The guest still
sees IDE or SATA but instead of translating to iSCSI the I/O requests
are sent over vhost-user-blk.

I suggest joining the libblkio chat and we can discuss how to set this
up (the QEMU libblkio BlockDriver is not yet in qemu.git):
https://matrix.to/#/#libblkio:matrix.org

> We are planning to start a review of these options internally to survey
> tradeoffs, potential timelines and practicality for these approaches. We
> were also considering putting a submission together for KVM forum
> describing our findings. Would you see any value in that?

I think it's always interesting to see performance results. I wonder if
you have more cutting-edge optimizations or performance results you want
to share at KVM Forum because IDE and SATA are more legacy/niche
nowadays?

Stefan

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