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

[Alexander Graf]

On 26.05.2010, at 21:28, Stefan Hajnoczi wrote:

> 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.

Well since cache=writethrough is the default, it apparently isn't hit by the 
issues you mentioned.

Kevin, what's your opinion on this?


Alex