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Re: [libvirt] [PATCH 3/3] qemu: support metadata-cache-size for blockdev



Am 02.11.2018 um 12:37 hat Nikolay Shirokovskiy geschrieben:
> On 02.11.2018 13:23, Kevin Wolf wrote:
> > Am 01.11.2018 um 12:32 hat Nikolay Shirokovskiy geschrieben:
> >> Just set l2-cache-size to INT64_MAX for all format nodes of
> >> qcow2 type in block node graph.
> >>
> >> AFAIK this is sane because *actual* cache size depends on size
> >> of data being referenced in image and thus the total size of
> >> all cache sizes for all images in disk backing chain will not
> >> exceed the cache size that covers just one full image as in
> >> case of no backing chain.
> > 
> > This is not quite correct.
> > 
> > Starting from qemu 3.1, INT64_MAX will add a cache that covers the whole
> > image. Memory is only used if a cache entry is actually used, so if you
> > never access the backing file, it doesn't really use any memory.
> > However, the granularity isn't a single cluster here, but L2 tables. So
> > if some L2 table contains one cluster in the overlay and another cluster
> > in the backing file, and both are accessed, the L2 table will be cached
> > in both the overlay and th backing file.
> 
> So we can end up N times bigger memory usage for L2 cache in case
> of backing chain comparing to single image?

In the worst case, yes.

Kevin


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