On Wed, Jun 15, 2011 at 2:14 PM, Dmitry Konishchev <konishc...@gmail.com> wrote: > On Wed, Jun 15, 2011 at 4:02 PM, Stefan Hajnoczi <stefa...@gmail.com> wrote: >> 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? > > 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