Re: [Qemu-devel] Default cache mode

[Stefan Hajnoczi]

On Wed, Jun 29, 2011 at 3:13 PM, Anthony Liguori <address@hidden> wrote:
> On 06/29/2011 08:50 AM, Christoph Hellwig wrote:
>>
>> On Wed, Jun 29, 2011 at 07:23:31AM -0500, Anthony Liguori wrote:
>>>
>>> Which file system on the host?
>>
>> Any filesystem.  Although extN and btrfs are particularly bad.
>>
>>> At any rate, I'm a big fan of making wce tunable in the guest and then I
>>> think setting wce=1 is quite reasonable to do by default.
>>
>> Note that this means a manual override during each boot.  Windows and
>> modern Linux just use what's available.  Only Solaris with ZFS actually
>> turns WCE on if it's not set AFAIK.  And to me it's a rather questionable
>> behaviour at least for SCSI disks where the admin might have chosen
>> those intentionally.  (unlike ATA disks SCSI allows to store the setting
>> permanently)
>
> Is WCE ever persisted by a disk?  Would it make sense to have a mechanism to
> persist the WCE setting for a guest?

Disks may store it in non-volatile memory, but it depends on the disk
and is reported in the cache mode page for SCSI.

Stefan