Re: [Qemu-devel] [PATCH v2 1/2] qcow2: Give the refcount cache the minimum possible size by default

[Alberto Garcia]

On Mon 16 Apr 2018 04:05:21 PM CEST, Max Reitz wrote:
>> Refcount entries are used to reference host clusters, and host
>> clusters are always(*) allocated sequentially, so caching refcount
>> blocks doesn't do much. You're always loading the same refcount block
>> until it's full, then you move on to the next one. As I mentioned in
>> a previous e-mail, having a very large refcount cache would even be
>> detrimental because it would make cache hits slower (noticeable under
>> tmpfs).
>
> Ah, right.  I'm not sure whether I want to ask you whether you have
> tested internal snapshots.  I suppose it can be detrimental for them
> because when taking or deleting a snapshot you need to update the
> refcounts of a whole bunch of clusters that might be spread randomly
> across the image.  But I suspect that it would be rather
> time-consuming to produce an image with such a configuration; and I
> suppose if people want to make heavy use of internal snapshots they
> can adapt the cache size themselves?

Exactly.

Berto