Re: [Qemu-block] [PATCH] qcow2: Optimize L2 table cache size based on image and cluster sizes

[Alberto Garcia]

On Wed 05 Oct 2016 05:35:20 PM CEST, Max Reitz <address@hidden> wrote:

>>> Apart from that, I have to say I think it would be a bit more useful
>>> if one would specify the area covered by the metadata caches as an
>>> absolute number instead of a relative one (I guess it's generally
>>> easier to know what area your applications will perform random
>>> accesses on than the relative size, but maybe that's just me).
>> 
>> I'm not sure if I'm following you, can you give an example of what
>> the area covered by the cache exactly means?
>
> Let's take the L2 table cache. Every eight bytes of that cache point
> to one cluster in the image, so if it's 8 MB in size, for instance,
> then you can cover 1M clusters. With the default 64 kB clusters, you
> would cover 64 GB of any image (although you have some constraints on
> that range, of course; an 8 MB cache would contain not just 1M cluster
> pointers but actually 128 L2 tables (8 MB / 64 kB), so it would
> actually cover 128 continuous 512 MB ranges).

Ah! You mean that instead of saying l2-cache-size=1M, you would say
l2-cache-coverage=8G, so the user doesn't need to do the numbers. I
thought you were talking about something different, that's why it didn't
make any sense to me :-)

Well, I don't know, if we had to design the interface again from the
beginning I guess it could make sense, but I don't think it's a good
idea to have two ways to specify exactly the same thing, when one can be
converted to the other with a very simple operation.

I admit the formula is not directly obvious (that's why I documented it
in docs/qcow2-cache.txt), but it's very simple.

I guess it wouldn't be so bad if we restricted it only to the command
line (for humans), since any program that communicates using QMP can do
the numbers itself. I don't know... how about a helper script that
checks the cluster size and tells you how much cache you need?

Berto