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

On Mon, 2 Apr 2012, Ian Campbell 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.
>               * Leverage XCP's blktap2 DKMS work.
>               * Other ideas?
>                       * In general we should recommend blkback (e.g.
>                         with LVM) since it always out performs other
>                         solutions, although at the expense of
>                         flexibility regarding image formats.
>         Stefano has done a lot of work here and has proposed some
>         performance improvements for qdisk in both qemu-xen and
>         qemu-xen-traditional which make them a plausible alternative for
>         Xen 4.2.

Using O_DIRECT rather than the default write-through cache policy solves
the performance problem in QEMU.
I sent patches to xen-devel to enable O_DIRECT for QDISK on
qemu-xen-traditional. I also sent patches to qemu-devel to enable
O_DIRECT and native AIO for QDISK on upstream QEMU.

Upstream QEMU PV backends are as good as the ones in
qemu-xen-traditional, but upstream QDISK performs better because it can
use native AIO. Thus I sent a patch to xen-devel to enable upstream QEMU
as default to provide userspace backends to PV guests.

I wrote and sent a patch series to fix the m2p override in Linux in case
the frontend and backend are both in the same domain.
Then I sent a patch series to xen-devel to make
libxl__device_disk_local_attach work with QDISK: the implementation uses
a frontend/backend pair in dom0.

As a result, with all these patches applied, the disk performances with
file based disk images are good and one can use a qcow2 file to store
a PV guest disk image and boot from it using pygrub.

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