12.07.2010 09:24, Giangiacomo Mariotti wrote: > Hi, is it a known problem how much slow is Btrfs with kvm/qemu(meaning > that the image kvm/qemu uses as the hd is on a partition formatted > with Btrfs, not that the fs used by the hd inside the kvm environment > is Btrfs, in fact inside kvm the / partition is formatted with ext3)? > I haven't written down the exact numbers, because I forgot, but while > I was trying to make it work, after I noticed how much longer than > usual it was taking to just install the system, I took a look at iotop > and it was reporting a write speed of the kvm process of approximately > 3M/s, while the Btrfs kernel thread had an approximately write speed > of 7K/s! Just formatting the partitions during the debian installation > took minutes. When the actual installation of the distro started I had > to stop it, because it was taking hours! The iotop results made me > think that the problem could be Btrfs, but, to be sure that it wasn't > instead a kvm/qemu problem, I cut/pasted the same virtual hd on an > ext3 fs and started kvm with the same parameters as before. The > installation of debian inside kvm this time went smoothly and fast, > like normally it does. I've been using Btrfs for some time now and > while it has never been a speed champion(and I guess it's not supposed > to be one and I don't even really care that much about it), I've never > had any noticeable performance problem before and it has always been > quite stable. In this test case though, it seems to be doing very bad. 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 -- 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
