linux-kernel - [PATCH 0/5] RFC: ->make_request support for virtio-blk

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Date:	Wed, 05 Oct 2011 15:54:03 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Chris Wright <chrisw@...s-sol.org>, Jens Axboe <axboe@...nel.dk>,
	Stefan Hajnoczi <stefanha@...ux.vnet.ibm.com>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
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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