Re: [Qemu-devel] [PATCH] [RFC] qcow2: group refcount updates during cow

[Kevin Wolf]

Am 28.07.2011 15:50, schrieb Frediano Ziglio:
> Well, I think this is the first real improve patch.
> Is more a RFC than a patch. Yes, some lines are terrible!
> It collapses refcount decrement during cow.
> From a first check time executing 015 test passed from about 600 seconds
> to 70.
> This at least prove that refcount updates counts!
> Some doubt:
> 1- place the code in qcow2-refcount.c as it update only refcount and not
>   cluster?
> 2- allow some sort of "begin transaction" / "commit" / "rollback" like 
>   databases instead?
> 3- allow changing tables from different coroutines?
> 
> 1) If you have a sequence like (1, 2, 4) probably these clusters are all in
> the same l2 table but with this code you get two write instead of one.
> I'm thinking about a function in qcow2-refcount.c that accept an array of 
> cluster
> instead of a start + len.
> 
> Signed-off-by: Frediano Ziglio <address@hidden>

I think what you're seeing is actually just one special case of a more
general problem. The problem is that we're interpreting writethrough
stricter than required.

The semantics that we really need is that on completion of a request,
all of its data and metadata must be flushed to disk. There is no
requirement that we flush all intermediate states.

My recent update to qcow2_update_snapshot_refcount() is just another
case of the same problem. I think the solution should be similar to what
I did there, i.e. switch the cache to writeback mode while we're
operating on it and switch back when we're done. We should probably have
functions that make both of this a one-liner (I think here we have some
similarity to your begin/commit idea).

With the right functions, this could become as easy as this (might need
better function names, but you get the idea):

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 882f50a..45b67b1 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -612,6 +612,8 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState
*bs, QCowL2Meta *m)
     if (m->nb_clusters == 0)
         return 0;

+    qcow2_cache_disable_writethrough(bs);
+
     old_cluster = qemu_malloc(m->nb_clusters * sizeof(uint64_t));

     /* copy content of unmodified sectors */
@@ -683,6 +685,7 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState
*bs, QCowL2Meta *m)

     ret = 0;
 err:
+    qcow2_cache_restore_writethrough(bs);
     qemu_free(old_cluster);
     return ret;
  }

Kevin