Re: [libvirt] [Qemu-devel] QEMU interfaces for image streaming and post-copy block migration
On 09/07/2010 05:57 PM, Anthony Liguori wrote:
> I agree that streaming should be generic, like block migration.
The
> trivial generic implementation is:
>
> void bdrv_stream(BlockDriverState* bs)
> {
> for (sector = 0; sector< bdrv_getlength(bs); sector += n) {
> if (!bdrv_is_allocated(bs, sector,&n)) {
Three problems here. First problem is that bdrv_is_allocated is
synchronous.
Put the whole thing in a thread.
The second problem is that streaming makes the most sense when
it's
the smallest useful piece of work whereas bdrv_is_allocated() may
return a very large range.
You could cap it here but you then need to make sure that cap is at
least cluster_size to avoid a lot of unnecessary I/O.
That seems like a nice solution. You probably want a multiple of the
cluster size to retain efficiency.
The QED streaming implementation is 140 LOCs too so you quickly end up
adding more code to the block formats to support these new interfaces
than it takes to just implement it in the block format.
bdrv_is_allocated() already exists (and is needed for commit), what else
is needed? cluster size?
Third problem is that streaming really requires being able to do
zero
write detection in a meaningful way. You don't want to always do zero
write detection so you need another interface to mark a specific write
as a write that should be checked for zeros.
You can do that in bdrv_stream(), above, before the actual write, and
call bdrv_unmap() if you detect zeros.
--
error compiling committee.c: too many arguments to function