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Re: [Xen-devel] Xen 4.2 Release Plan / TODO

On Mon, 2012-03-19 at 11:25 +0000, Jan Beulich wrote:
> >>> On 19.03.12 at 11:57, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> >       * file:// backend performance. qemu-xen-tradition's qdisk is quite
> >         slow & blktap2 not available in upstream kernels. Need to
> >         consider our options:
> >               * qemu-xen's qdisk is thought to be well performing but
> >                 qemu-xen is not yet the default. Complexity arising from
> >                 splitting qemu-for-qdisk out from qemu-for-dm and
> >                 running N qemu's.
> >               * potentially fully userspace blktap could be ready for
> >                 4.2
> >               * use /dev/loop+blkback. This requires loop driver AIO and
> >                 O_DIRECT patches which are not (AFAIK) yet upstream.
> I meant to ask already when this was first mentioned: What's the
> reason for this requirement? Didn't we have blkback over loop running
> fine for years? Or is this just a performance consideration (in which
> case "requires" might be too strong a term)?

My understanding (which could well be totally bogus) was that the use
of /dev/loop in this way was unsafe since pages were only committed to
the dom0 page cache and not to the actual platter when success was
reported to the guest. I think that is why many people used tap:aio:
instead of file: (personally I use phy: almost exclusively so I could be
talking rubbish).

Unless there are some loop patches in the classic-Xen patchset? I don't
think there are though.

I don't know so much about the performance aspect. Stefano might be able
to comment.


> Jan
> >               * Leverage XCP's blktap2 DKMS work.
> >               * Other ideas?

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