From: Kevin Wolf <kwolf@redhat.com>
To: Slava Pestov <sviatoslavpestov@gmail.com>
Cc: Stefan Hajnoczi <stefanha@gmail.com>,
Andrey Korolyov <andrey@xdel.ru>,
qemu-devel <qemu-devel@nongnu.org>,
Stefan Hajnoczi <stefanha@redhat.com>
Subject: Re: [Qemu-devel] [PATCH 2/2] qemu-iotests: add multiwrite test cases
Date: Fri, 1 Aug 2014 12:02:06 +0200 [thread overview]
Message-ID: <20140801100205.GD4446@noname.redhat.com> (raw)
In-Reply-To: <CALQsL_Y_6RbrYarR+6wN=x1jym813m5V4Yf_gzLiCiP4Y-csdQ@mail.gmail.com>
Am 31.07.2014 um 20:19 hat Slava Pestov geschrieben:
> Why are you guys merging requests in qemu at all? Just submit them to the
> kernel and let the kernel do it.
Because the kernel generally isn't the next one seeing the requests. You
might be right for the special case of using only raw images with
cache=none,aio=native, where we would theoretically have a chance to
submit all requests in a batch with a single syscall and then let the
kernel merge them. It just isn't what everyone is running.
An important case is requests going to non-raw image format drivers,
where many small writes on sparse images are inefficient because they
cause a lot of unnecessary COW activity. Another case are backends that
don't even send the requests to the kernel, but use e.g. a network
protocol as their backend.
Kevin
prev parent reply other threads:[~2014-08-01 10:02 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-07-29 12:41 [Qemu-devel] [PATCH 0/2] block: fix multiwrite_merge() overlapping requests Stefan Hajnoczi
2014-07-29 12:41 ` [Qemu-devel] [PATCH 1/2] block: fix overlapping multiwrite requests Stefan Hajnoczi
2014-07-29 12:46 ` Fam Zheng
2014-07-29 12:52 ` Stefan Hajnoczi
2014-07-29 12:41 ` [Qemu-devel] [PATCH 2/2] qemu-iotests: add multiwrite test cases Stefan Hajnoczi
2014-07-29 15:11 ` Eric Blake
2014-07-30 8:11 ` Stefan Hajnoczi
2014-07-31 18:19 ` Slava Pestov
2014-08-01 10:02 ` Kevin Wolf [this message]
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