[Sheepdog] [PATCH v5 4/8] sheep: teach sheep to use object cache

[MORITA Kazutaka]

At Mon, 26 Mar 2012 12:15:29 +0800,
Liu Yuan wrote:
> 
> On 03/26/2012 04:41 AM, MORITA Kazutaka wrote:
> 
> > Consider that the object is dirty if the VDI was opened with a
> > writeback mode in the previous time.  I think we need to check a cache
> > here.
> 
> 
> For the sane usage, VMs should not be opened both for none-cache and
> cache mode off and on. Even if VM first open the vdi with cache mode
> first, then want to use none-cache mode, current implementation would be
> okay, since after the guest was closed, the dirty bits would be flushed
> to the cluster.
> 
> So the problem is precisely that we can't handle the situation that VM
> runs with cache first, then crashes, then boots up again without cache.
> We'd better use qemu-io to issue a flush requests first manually, then
> we can safe boot the VM without cache, not losing any dirty data.
> 
> So I don't think it worth introducing complexity to object cache to
> handle this corner case that already should be handled by other means.

We can more easily hit this problem.  For example:

 $ qemu-img convert linux.raw sheepdog:linux

where linux.raw is a linux image file, and

 $ qemu sheepdog:linux

We cannot boot the linux VDI because qemu-img opens with a
BDRV_O_CACHE_WB always, but qemu opens without it by default.

I think you are mixing two things in this patchset.
  1. use a disk cache (this should be disabled by BDRV_O_NOCACHE)
  2. support a writeback mode (this should be enabled by BDRV_O_CACHE_WB)

If you think it doesn't make sense to support a writethrough mode with
a disk cache, I'm not against to the current approach in this
patchset.  But at least, please flush a dirty cache before handling
non-cache requests.  Otherwise, we can easily hit a cache coherency
problem (e.g. 'collie vdi list' doesn't show the correct used size).

Thanks,

Kazutaka