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Date: Wed, 15 Jun 2011 14:33:22 +0100
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From: Stefan Hajnoczi <stefanha@gmail.com>
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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 2:14 PM, Dmitry Konishchev <konishchev@gmail.com> w=
rote:
> On Wed, Jun 15, 2011 at 4:02 PM, Stefan Hajnoczi <stefanha@gmail.com> wro=
te:
>> We need to fully understand performance before applying optimizations
>> on top. =A0Otherwise it is possible to paper over a problem while
>> leaving the root cause unsolved. =A0Avoiding lseek(2) is very important,
>> not just for qemu-img but also for running VMs. =A0lseek(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=3D/dev/vda of=3D/dev/null iflag=3Ddirect
>>
>> I see pread(2) from the posix-aio-compat.c worker threads but no
>> lseek(2). =A0The total_sectors cached value is being used.
>>
>> Does strace(1) show lseek(2) on your host?
>
> No, I don't see lseek() in the strace output. But in the other hand I
> see that bdrv_get_geometry() uses bdrv_getlength() which is
> implemented using lseek() in block/raw-posix.c...
> May be I'am mistaken about lseek(), but I get 9% slower version if
> disable caching.

"disable caching"?

Stefan