Re: [Qemu-devel] [Qemu-block] [PATCH] qcow2: do lazy allocation of the L2 cache

[Stefan Hajnoczi]

On Tue, Apr 21, 2015 at 06:20:39PM +0300, Alberto Garcia wrote:
> Large disk images need large L2 caches in order to maximize their
> I/O performance. However setting a correct size for the cache is not
> necessarily easy since apart from the image size, it also depends
> on other factors like its usage patterns or whether it's part of a
> backing chain.
> 
> In order to be able to set a very large cache size to cover the
> worst-case scenarios and yet prevent an unnecessary waste of memory,
> this patch modifies the qcow2 cache algorithm so the memory for each
> entry is allocated only when it's actually needed.
> 
> This also improves the scenarios with smaller images: the current
> default L2 cache size can map a whole 8GB disk, so those smaller than
> that are allocating cache memory that can never be used.

What measurable improvement does this patch make?

Buffers allocated upfront are not touched, so they don't actually
consume physical memory.

Stefan

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