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. -- Дмитрий Конищев (Dmitry Konishchev) mailto:konishc...@gmail.com
