Re: [Qemu-devel] [PATCH] Default to cache=writeback

[Stefan Hajnoczi]

On Wed, May 26, 2010 at 7:13 PM, Alexander Graf <address@hidden> 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.

Stefan