Re: [Qemu-block] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk

[Alberto Garcia]

On Tue 13 Dec 2016 09:02:34 AM CET, Max Reitz wrote:

> I definitely like how simple your approach is, but from a design
> standpoint it is not exactly optimal, because O(n) for a cluster
> lookup is simply worse than O(1).

It's actually not O(n) anymore since I rewrote that code last year.

When I changed the algorithm to use LRU for cache eviction I also
optimized the lookup:

 1) qcow2_cache_put() simply divides the table offset by the cluster
    size and it immediately obtains the index.

 2) qcow2_cache_get() hashes the offset to do the lookup. In my tests
    doing random I/O and a 2048 entry cache the average number of
    comparisons was 2.5 (430 before the rewrite).

The cache was ~13% faster after the rewrite, but the results in terms of
actual disk I/O improvements were negligible, because that's not where
the bottleneck is. We even discussed how the hash function that I was
using was weak, but we decided not to touch it because it was not worth
the effort.

>> But leaving that aside, would that improve anything? I don't think
>> the cache itself adds any significant overhead here, IIRC even in
>> your presentation at KVM Forum 2015 qcow2 was comparable to raw as
>> long as all L2 tables were cached in memory.
>
> I haven't compared CPU usage, though. That may have gone up quite a
> bit, I don't know. For large enough images, it may even become a
> bottleneck.

I don't know, I don't have any data to suggest that there's a problem
there, particularly after the rewrite from last year I think the cache
is reasonably fast (as long as the cache is big enough; and if it's not
big enough the problem is going to be in the extra I/O).

Berto