[libvirt] PATCH: Disable QEMU drive caching
Daniel P. Berrange
berrange at redhat.com
Wed Oct 8 17:05:39 UTC 2008
On Wed, Oct 08, 2008 at 11:49:19AM -0500, Anthony Liguori wrote:
> Daniel P. Berrange wrote:
> >On Wed, Oct 08, 2008 at 11:06:27AM -0500, Anthony Liguori wrote:
> >
> >Sorry, it was mistakenly private - fixed now.
> >
> >Xen does use O_DIRECT for paravirt driver case - blktap is using the combo
> >of AIO+O_DIRECT.
>
> You have to use O_DIRECT with linux-aio. And blktap is well known to
> have terrible performance. Most serious users use blkback/blkfront and
> blkback does not avoid the host page cache. It maintains data integrity
> by passing through barriers from the guest to the host. You can
> approximate this in userspace by using fdatasync.
>
> The issue the bug addresses, iozone performs better than native, can be
> addressed in the following way:
>
> 1) For IDE, you have to disable write-caching in the guest. This should
> force an fdatasync in the host.
> 2) For virtio-blk, we need to implement barrier support. This is what
> blkfront/blkback do.
> 3) For SCSI, we should support ordered queuing which would result in an
> fdatasync when barriers are injected.
Ok, ignore my libvirt patch then. We'll punt this problem back to QEMU
& virtio develoers to solve properly
Daniel
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