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Subject[PATCH 0/5] RFC: ->make_request support for virtio-blk
This patchset allows the virtio-blk driver to support much higher IOP
rates which can be driven out of modern PCI-e flash devices. At this
point it really is just a RFC due to various issues.

The first four patches are infrastructure that could go in fairly
soon as far as I'm concerned. Patch 5 implements the actual ->make_request
support and still has a few issues, see there for more details. With
it I can driver my PCI-e test devices to 85-90% of the native IOPS
and bandwith, but be warned that this is still a fairly low end setup
as far as expensive flash storage is concerned.

One big downside that is has is that it current exposes a nasty race
in the qemu virtqueue code - just running xfstests inside a guest
using the new virtio-blk driver (even on a slow device) will trigger
it and lead to a filesystem shutdown. I've tracked it down to getting
data I/O segments overwritten with status s/g list entries, but got
lost at that point. I can start a separate thread on it.

Besides that it is missing a few features, and we have to decided
how to select which mode to use in virtio-blk - either a module option,
sysfs attribute or something that the host communicates. Or maybe
decide that just going with ->make_request alone is fine, even on
my cheap laptop SSD it actually is just as fast if not slightly
faster than the request based variant on my laptop.

There are a few other bottlenecks in virtio that this exposes. The
first one is the low queue length of just 128 entries in the virtio-blk
queue - to drive higher IOPs with a deep queue we absolutely need
to increment that.

Comments welcome!

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