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Date: Tue, 20 Apr 2010 10:35:05 +0200
From: Kevin Wolf <kwolf@redhat.com>
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Subject: [Qemu-devel] Re: [PATCH v2] block: Cache total_sectors to reduce
 bdrv_getlength calls
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To: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>, qemu-devel@nongnu.org, Christoph Hellwig <hch@lst.de>

Am 19.04.2010 17:56, schrieb Stefan Hajnoczi:
> The BlockDriver bdrv_getlength function is called from the I/O code path
> when checking that the request falls within the device.  Unfortunately
> this involves an lseek system call in the raw protocol; every read or
> write request will incur this lseek cost.
> 
> Jan Kiszka <jan.kiszka@siemens.com> identified this issue and its
> latency overhead.  This patch caches device length in the existing
> total_sectors variable so lseek calls can be avoided for fixed size
> devices.
> 
> Growable devices fall back to the full bdrv_getlength code path because
> I have not added logic to detect extending the size of the device in a
> write.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
> ---
> v2:
> - Introduced refresh_total_sectors() to clean up total_sectors updates
> - Added error handling for refresh_total_sectors() callers

Looks good, applied to the block branch.

Kevin