[Sheepdog] [PATCH v6 7/8] collie: read the cached objects if any for collie operation

[MORITA Kazutaka]

At Thu, 05 Apr 2012 15:28:10 +0800,
Liu Yuan wrote:
> 
> On 03/31/2012 06:23 PM, Liu Yuan wrote:
> 
> > On 03/31/2012 05:29 PM, MORITA Kazutaka wrote:
> > 
> >> At Sat, 31 Mar 2012 15:34:10 +0800,
> >> Liu Yuan wrote:
> >>>
> >>> On 03/31/2012 03:20 PM, MORITA Kazutaka wrote:
> >>>
> >>>> - Read requests:
> >>>>   read from a object cache always when the cache exists
> >>>>
> >>>> - Write requests:
> >>>>   flush and remove a object cache if SD_FLAG_CMD_CACHE is not set
> >>>>
> >>>> Isn't this okay for collie, too?
> >>>
> >>>
> >>> Who set SD_FLAG_CMD_CACHE for collie? user or let collie decide itself?
> >>
> >> We don't need to set the same cache flag to the collie requests.
> >>
> >> Collie doesn't send write requests to the opened VDIs, so collie
> >> doesn't flush the existing caches with my idea.
> >>
> 
> 
> I think this assumption is broken, collie vdi snapshot will do WRITE the
> other's opened VDI....

The only way to take a snapshot of running VMs is a savevm command
from the qemu monitor.  We can use neither collie nor qemu-img for it.

> 
> So collie vdi snapshot will simply remove the object cache of the victim
> Guest... this is quite costy operation.
> 
> So I'd like insisting that, we'd better not try to fix the bug that
> should be fixed elsewhere. I think qemu-img convert should use
> cache=passthrough mode, it is unfriendly for it to use writeback cache.

It takes much longer time to convert VM disks with a writethrough
mode.  I'm not sure such change is acceptable for QEMU community.

Thanks,

Kazutaka