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Re: [Qemu-devel] KVM call agenda for October 25
From: |
Daniel P. Berrange |
Subject: |
Re: [Qemu-devel] KVM call agenda for October 25 |
Date: |
Wed, 26 Oct 2011 12:39:32 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Wed, Oct 26, 2011 at 01:23:05PM +0200, Kevin Wolf wrote:
> Am 26.10.2011 11:57, schrieb Daniel P. Berrange:
> > On Wed, Oct 26, 2011 at 10:48:12AM +0200, Markus Armbruster wrote:
> >> Kevin Wolf <address@hidden> 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
>
> I'm not sure if a 4k host should imply a 4k guest by default. This means
> that some guests wouldn't be able to run on a 4k host. On the other
> hand, for those guests that can do 4k, it would be the much better option.
>
> So I think this decision is the hard thing about it.
I guess it somewhat depends whether we want to strive for
1. Give the user the fastest working config by default
2. Give the user a working config by default
3. Give the user the fastest (possibly broken) config by default
IMHO 3 is not a serious option, but I could see 2 as a reasonable
tradeoff to avoid complexity in chosing QEMU defaults. The user
would have a working config with 512 sectors, but sub-optimal perf
on 4k hosts due to bounce buffering. Ideally libvirt or other
higher app would be setting the best block size that a guest
can support by default, so bounce buffers would rarely be needed.
So only people using QEMU directly without setting a block size
would ordinarily suffer the bounce buffer perf hit on a 4k host
host
Daniel
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- Re: [Qemu-devel] KVM call agenda for October 25, (continued)
- Re: [Qemu-devel] KVM call agenda for October 25, Kevin Wolf, 2011/10/26
- Re: [Qemu-devel] KVM call agenda for October 25, Markus Armbruster, 2011/10/26
- Re: [Qemu-devel] KVM call agenda for October 25, Paolo Bonzini, 2011/10/26
- Re: [Qemu-devel] KVM call agenda for October 25, Markus Armbruster, 2011/10/26
- Re: [Qemu-devel] KVM call agenda for October 25, Daniel P. Berrange, 2011/10/26
- Re: [Qemu-devel] KVM call agenda for October 25, Kevin Wolf, 2011/10/26
- Re: [Qemu-devel] KVM call agenda for October 25,
Daniel P. Berrange <=
- Re: [Qemu-devel] KVM call agenda for October 25, Kevin Wolf, 2011/10/26