On Wed, Jun 15, 2011 at 10:50 AM, Dmitry Konishchev <konishc...@gmail.com> wrote: > On Wed, Jun 15, 2011 at 12:39 PM, Stefan Hajnoczi <stefa...@gmail.com> wrote: >> Why is bdrv_get_geometry() slow? > > Mmm.. Frankly, I haven't looked so deep, but it is going to be slow at > least for raw images due to using lseek().
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? Stefan
