Re: [Qemu-devel] Adding a persistent writeback cache to qemu

[Stefan Hajnoczi]

On Mon, Apr 01, 2013 at 01:21:45PM +0000, Alex Bligh wrote:
> I'd like to experiment with adding persistent writeback cache to qemu.
> The use case here is where non-local storage is used (e.g. rbd, ceph)
> using the qemu drivers, together with a local cache as a file on
> a much faster locally mounted device, for instance an SSD (possibly
> replicated). This would I think give a similar performance boost to
> using an rbd block device plus flashcache/dm-cache/bcache, but without
> introducing all the context switches and limitations of having to
> use real block devices. I appreciate it would need to be live migration
> aware (worst case solution: flush and turn off caching during live
> migrate), and ideally be capable of replaying a dirty writeback cache
> in the event the host crashes.
> 
> Is there any support for this already? Has anyone worked on this before?
> If not, would there be any interest in it?

I'm concerned about the complexity this would introduce in QEMU.
Therefore I'm a fan of using existing solutions like the Linux block
layer instead of reimplementing this stuff in Linux.

What concrete issues are there with using rbd plus
flashcache/dm-cache/bcache?

I'm not sure I understand the context switch problem since implementing
it in user space will still require system calls to do all the actual
cache I/O.

Stefan