On Tue, Jun 1, 2010 at 12:31 PM, Alexander Graf <ag...@suse.de> wrote: > > On 26.05.2010, at 21:28, Stefan Hajnoczi wrote: > >> On Wed, May 26, 2010 at 7:13 PM, Alexander Graf <ag...@suse.de> wrote: >>> In the previous discussion Anthony brought up the fact that cache=writeback >>> is >>> safe enough considering data integrity. If so, I don't see a reason not to >>> use >>> it as default, as it speeds up things a lot. >> >> cache=writeback is not a good default for qcow2: >> >> https://bugzilla.redhat.com/show_bug.cgi?id=572825 >> http://wiki.qemu.org/Features/Qcow2DataIntegrity >> >> The actual qcow2 file itself can become corrupted because metadata >> updates are not ordered or flushed with respect to each other or data. >> This is more serious than losing data written after the last flush. >> >> I believe special case cache= defaults for raw vs qcow2 have been >> mentioned before but I don't see any code in qemu.git currently that >> ensures qcow2 is run safely by default. > > Well since cache=writethrough is the default, it apparently isn't hit by the > issues you mentioned.
cache=writethrough is safe because it uses O_SYNC. Stefan
