Re: [Qemu-devel] Using cache=writeback safely on qemu 1.4.0 and later

[Andrew Martin <amartin@xes-inc.com>]

----- Original Message -----
> From: "Paolo Bonzini" <pbonzini@redhat.com>
> To: "Andrew Martin" <amartin@xes-inc.com>, "Stefan Hajnoczi" <stefanha@gmail.com>
> Cc: "qemu-devel" <qemu-devel@nongnu.org>
> Sent: Tuesday, August 26, 2014 2:03:18 AM
> Subject: Re: Using cache=writeback safely on qemu 1.4.0 and later
> 
> > In the case of the files uploaded by apache2, they were added to the server
> > days
> > before  the power outage, so it seems like there would have been ample time
> > for
> > those changes to have been flushed.
> 
> Remember that cache=writeback has a huge cache, whose size is several
> gigabytes.
> 
> At some point there was a QEMU bug that caused the guest not to do
> fsyncs.  It was fixed by commit ef5bc96268.

Paolo,

Thanks for the info about this bug - I am using QEMU 1.4.0 (released on 02/15/13)
and ef5bc96268 was added on 09/20/13, so the bug had not been fixed in the version
of QEMU I am using. I have had trouble reproducing it on-demand - was there specific
behavior you used to reproduce it, or just the lack of fsyncs or fdatasyncs when
stracing the QEMU process when using cache=writeback?

Thanks,

Andrew