block: Cache total_sectors to reduce bdrv_getlength calls
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: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com> Signed-off-by: NKevin Wolf <kwolf@redhat.com>
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