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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


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