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Re: [libvirt] [PATCH 0/2] Add 'l2-cache-size' property to specify maximum size of the L2 table cache for qcow2 image



On Wed 15 Nov 2017 12:21:24 PM CET, Kashyap Chamarthy wrote:
> [Cc: Alberto Garcia of QEMU, who added the 'l2-cache-size' upstream]

[...]

>> There's already something on list about l2 cache, see
>> 
>> https://www.redhat.com/archives/libvir-list/2017-September/msg00553.html
>> 
>> it adds 3 related variables. IIRC, the "hold up" is how best to
>> describe how to use it or provide the feature in such a way that it's
>> "understandable" for general consumption without making a "policy"
>> that generates certain values based on expected usage/needs for the
>> client.
>
> I recall Alberto saying that there's a default value of QCOW2
> 'l2-cache-size', but it is (intentionally?) low.  And that the value
> depends on the workload / use case: size of the image, the amount of
> I/O you want to do, etc.

I actually didn't add that parameter.

Looking at the history it seems that the default size hasn't changed
since Fabrice Bellard added it back in 2006, so I assume that it was
fine for most use cases back in the day and it was never discussed
afterwards.

This was probably mentioned already, but here's what I've written about
this topic:

   https://bugzilla.redhat.com/show_bug.cgi?id=1377735#c2

   https://www.redhat.com/archives/libvir-list/2017-October/msg00180.html

>From the user's point of view the simplest use cases are perhaps:

   1) I want to use at most xxx MB for the qcow2 L2 cache.
   2) I want to use as much cache as I need in order to get the best
      performance.

Those should be relatively straightforward and easy to understand for
the user, (1) is simply the value of l2-cache-size, and for (2) libvirt
would have to make the calculations itself.

Some things to take into account:

   - l2-refcount-size is set to be 1/4 of l2-cache-size (see "How to
     configure the cache sizes" in docs/qcow2-cache.txt in QEMU), but it
     probably makes sense to keep it artificially low (it's not so
     important for I/O perfomance, although I dont have numbers of its
     actual impact).
   - cache-clean-interval can also be useful, see the bugzilla link for
     more details.

Berto


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