On Wed, Oct 26, 2011 at 10:48:12AM +0200, Markus Armbruster wrote:
> Kevin Wolf <kwolf@xxxxxxxxxx> writes:
>
> > Am 25.10.2011 16:06, schrieb Anthony Liguori:
> >> On 10/25/2011 08:56 AM, Kevin Wolf wrote:
> >>> Am 25.10.2011 15:05, schrieb Anthony Liguori:
> >>>> I'd be much more open to changing the default mode to cache=none FWIW since the
> >>>> risk of data loss there is much, much lower.
> >>>
> >>> I think people said that they'd rather not have cache=none as default
> >>> because O_DIRECT doesn't work everywhere.
> >>
> >> Where doesn't it work these days? I know it doesn't work on tmpfs. I know it
> >> works on ext[234], btrfs, nfs.
> >
> > Besides file systems (and probably OSes) that don't support O_DIRECT,
> > there's another case: Our defaults don't work on 4k sector disks today.
> > You need to explicitly specify the logical_block_size qdev property for
> > cache=none to work on them.
> >
> > And changing this default isn't trivial as the right value doesn't only
> > depend on the host disk, but it's also guest visible. The only way out
> > would be bounce buffers, but I'm not sure that doing that silently is a
> > good idea...
>
> Sector size is a device property.
>
> If the user asks for a 4K sector disk, and the backend can't support
> that, we need to reject the configuration. Just like we reject
> read-only backends for read/write disks.
I don't see why we need to reject a guest disk with 4k sectors,
just because the host disk only has 512 byte sectors. A guest
sector size that's a larger multiple of host sector size should
work just fine. It just means any guest sector write will update
8 host sectors at a time. We only have problems if guest sector
size is not a multiple of host sector size, in which case bounce
buffers are the only option (other than rejecting the config
which is not too nice).
IIUC, current QEMU behaviour is
Guest 512 Guest 4k
Host 512 * OK OK
Host 4k * I/O Err OK
'*' marks defaults
IMHO, QEMU needs to work withot I/O errors in all of these
combinations, even if this means having to use bounce buffers
in some of them. That said, IMHO the default should be for
QEMU to avoid bounce buffers, which implies it should either
chose guest sector size to match host sector size, or it
should unconditionally use 4k guest. IMHO we need the former
Guest 512 Guest 4k
Host 512 *OK OK
Host 4k OK *OK
Yes, I know there are other wierd sector sizes besides 512
and 4k, but the same general principals apply of either one
being a multiple of the other, or needing to use bounce
buffers.
> If the backend can only support it by using bounce buffers, I'd say
> reject it unless the user explicitly permits bounce buffers. But that's
> debatable.
I don't think it really adds value for QEMU to force the user to specify
some extra magic flag in order to make the user's requested config
actually be honoured. If a config needs bounce buffers, QEMU should just
do it, without needing 'use-bounce-buffers=1'. A higher level mgmt app
is in a better position to inform users about the consequences.
Daniel
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