On 2014-11-10 at 22:12, Eric Blake wrote:
On 11/10/2014 06:45 AM, Max Reitz wrote:
qcow2_alloc_bytes() may reuse a cluster multiple times, in which case
the refcount is increased accordingly. However, if this would lead to an
overflow the function should instead just not reuse this cluster and
allocate a new one.
So if recount_order is 1 (2 bits per refcount, max refcount of 4
*max refcount of 3 (0b11)
), and
we encounter the same cluster 6 times (say by 5 back-to-back internal
snapshots), does this code optimize to only 2 clusters (both with
refcount 3) or does it result in each of the last 3 clusters spilling to
its own 1-ref cluster for a total of 4 clusters? Short of Benoit's work
on deduplication, is there even a way to avoid inefficient use of
spilled clusters?
I'm not sure what you're referring to; maybe I should add that
qcow2_alloc_bytes() is used for allocating compressed clusters (which
ideally don't take up a full host cluster), so "reuse" in this context
just means that several compressed clusters share one host cluster.
Maybe you're referring to the following situation: We have the default
cluster size of 64k. Now we're trying to allocate 16k for each of the
compressed clusters A, B, C and D. D won't fit into that cluster because
the maximum refcount is three, so it will be put into a newly allocated
host cluster. Finally, we're trying to allocate 32k for a compressed
cluster E, which will then be put into the same cluster as D. We
therefore have the following allocation (each sub-box representing 16k):
+---+---+---+---+ +---+---+---+---+
|A |B | C | | | D | E | |
+---+---+---+---+ +---+---+---+---+
whereas the ideal allocation would be:
+---+---+---+---+ +---+---+---+---+
|A |B | E | | C | D | | |
+---+---+---+---+ +---+---+---+---+
This is a problem, but I think first it's a minor one (just use a
sufficiently large refcount width if you're going to use compressed
clusters) and second it's about compressed clusters, whose performance I
could hardly care less about, frankly.
Max
But I guess answering that can be a separate patch;
inefficiency is annoying, but not technically wrong and therefore not a
reason to reject this one.
Signed-off-by: Max Reitz <mre...@redhat.com>
---
block/qcow2-refcount.c | 32 ++++++++++++++++++++++++++++++--
1 file changed, 30 insertions(+), 2 deletions(-)
Reviewed-by: Eric Blake <ebl...@redhat.com>
