On Thu, Jun 16, 2011 at 9:10 AM, Dmitry Konishchev <konishc...@gmail.com> wrote: > On Wed, Jun 15, 2011 at 5:57 PM, Stefan Hajnoczi <stefa...@gmail.com> wrote: >> Anyway, bdrv_getlength() will return the total_sectors value instead >> of calling into raw-posix.c .bdrv_getlength(). That's why it should >> be cheap. > > Yeah, I see it now after a closer look in the drivers code. It looks > like I get this 9% slowdown just because for my particular test > bdrv_get_geometry() is called 37,348,536 times, which is a big number > even for function which calls another function which checks a few IFs > and returns a multiplication of two numbers.
Ultimately the QEMU block layer should make total_sectors always up-to-date so it can be fetched cheaply. This requires looking at all the block drivers and considering when device size may change at runtime. Your patch improves backing file convert cases without regressing other cases too much. If you continue optimizing qemu-img, consider profiling the block operations and improving the core block driver performance instead of the outer loop of the qemu-img command. That way running VMs will benefit from your optimizations too. And the simple outer loop may be acceptable if the operation inside it is fast enough. Kevin: Are you happy with this patch? Stefan
