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block: Give always priority to unused entries in the qcow2 L2 cache
The current algorithm to replace entries from the L2 cache gives priority to newer hits by dividing the hit count of all existing entries by two everytime there is a cache miss. However, if there are several cache misses the hit count of the existing entries can easily go down to 0. This will result in those entries being replaced even when there are others that have never been used. This problem is more noticeable with larger disk images and cache sizes, since the chances of having several misses before the cache is full are higher. If we make sure that the hit count can never go down to 0 again, unused entries will always have priority. Signed-off-by: Alberto Garcia <berto@igalia.com> Reviewed-by: Max Reitz <mreitz@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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