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

On Mon, 19 Mar 2012, 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)?

There are several problems here:

- the usage of loop with blkback is unsafe because loop doesn't support
O_DIRECT, at least the version of loop present in upstream Linux. Also
this means that the very good performance results that we get with loop
are actually inflated, the real numbers could be very low.

- Loop with blkback doesn't work with anything but raw files, so it
won't work for qcow, qcow2 or vhd.

- Using qdisk as backend, with or without AIO, is possible but from the
performance measurements I have run so far is very slow. In theory this
should be the best option, but in practice I cannot explain why I am
getting 1/10 of the performances I am expecting.

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