Giangiacomo Mariotti wrote: > On Mon, Jul 12, 2010 at 9:09 AM, Michael Tokarev <mjt@xxxxxxxxxx> wrote: >> This looks quite similar to a problem with ext4 and O_SYNC which I >> reported earlier but no one cared to answer (or read?) - there: >> http://permalink.gmane.org/gmane.linux.file-systems/42758 >> (sent to qemu-devel and linux-fsdevel lists - Cc'd too). You can >> try a few other options, esp. cache=none and re-writing some guest >> files to verify. >> >> /mjt >> > Either way, changing to cache=none I suspect wouldn't tell me much, > because if it's as slow as before, it's still unusable and if instead > it's even slower, well it'd be even more unusable, so I wouldn't be > able to tell the difference. Actually it's not that simple. > What I can say for certain is that with > the exact same virtual hd file, same options, same system, but on an > ext3 fs there's no problem at all, on a Btrfs is not just slower, it > takes ages. It is exactly the same with ext4 vs ext3. But only on metadata-intensitive operations (for qcow2 image). Once you allocate space, it becomes fast, and _especially_ fast with cache=none. Actually, it looks like O_SYNC (default cache mode) is _slower_ on ext4 than O_DIRECT (cache=none). (And yes, I know O_DIRECT does NOT imply O_SYNC and vise versa). /mjt -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
