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Date: Wed, 15 Jun 2011 13:02:01 +0100
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From: Stefan Hajnoczi <stefanha@gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Subject: Re: [Qemu-devel] [PATCH] CPU consumption optimization of 'qemu-img
 convert' using bdrv_is_allocated()
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To: Dmitry Konishchev <konishchev@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel@nongnu.org

On Wed, Jun 15, 2011 at 10:50 AM, Dmitry Konishchev
<konishchev@gmail.com> wrote:
> On Wed, Jun 15, 2011 at 12:39 PM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
>> Why is bdrv_get_geometry() slow?
>
> Mmm.. Frankly, I haven't looked so deep, but it is going to be slow at
> least for raw images due to using lseek().

We need to fully understand performance before applying optimizations
on top.  Otherwise it is possible to paper over a problem while
leaving the root cause unsolved.  Avoiding lseek(2) is very important,
not just for qemu-img but also for running VMs.  lseek(2) should only
be invoked if the image is growable/removable.

When I run a VM from a virtio-blk raw image I see no lseek(2) calls.

On the host:
strace -p $pid_of_qemu -f

Inside the guest:
dd if=/dev/vda of=/dev/null iflag=direct

I see pread(2) from the posix-aio-compat.c worker threads but no
lseek(2).  The total_sectors cached value is being used.

Does strace(1) show lseek(2) on your host?

Stefan