qemu/docs/qcow2-cache.txt

1 qcow2 L2/refcount cache configuration
12 performance significantly. However, setting the right cache sizes is
50 an L2 cache in memory to speed up disk access.
52 The size of the L2 cache can be configured, and setting the right
73 QEMU keeps a refcount cache to speed up I/O much like the
74 aforementioned L2 cache, and its size can also be configured.
77 Choosing the right cache sizes
79 In order to choose the cache sizes we need to know how they relate to
100 For example, 1MB of L2 cache is needed to cover every 8 GB of the virtual
105 The refcount cache is 4 times the cluster size by default. With the default
112 How to configure the cache sizes
119 "l2-cache-size":         maximum size of the L2 table cache
120 "refcount-cache-size":   maximum size of the refcount block cache
121 "cache-size":            maximum size of both caches combined
126    (or the cache entry size: see "Using smaller cache sizes" below).
128  - The maximum L2 cache size is 32 MB by default on Linux platforms (enough
130    value can be modified using the "l2-cache-size" option. QEMU will not use
134    this difference stems from the fact that on Linux the cache can be cleared
135    periodically if needed, using the "cache-clean-interval" option (see below).
136    The minimal L2 cache size is 2 clusters (or 2 cache entries, see below).
138  - The default (and minimum) refcount cache size is 4 clusters.
140  - If only "cache-size" is specified then QEMU will assign as much
141    memory as possible to the L2 cache before increasing the refcount
142    cache size.
144  - At most two of "l2-cache-size", "refcount-cache-size", and "cache-size"
150 refcount cache size won't have any measurable effect in performance
152 to think about increasing the cache size if you use them heavily).
154 Before QEMU 2.12 the refcount cache had a default size of 1/4 of the
155 L2 cache size. This resulted in unnecessarily large caches, so now the
156 refcount cache is as small as possible unless overridden by the user.
159 Using smaller cache entries
161 The qcow2 L2 cache can store complete tables. This means that if QEMU
163 and is kept in the cache. If the cache is full then a complete table
167 more disk I/O and wastes more cache memory.
169 Since QEMU 2.12 you can change the size of the L2 cache entry and make
171 "l2-cache-entry-size" parameter:
173    -drive file=hd.qcow2,l2-cache-size=2097152,l2-cache-entry-size=4096
175 Since QEMU 4.0 the value of l2-cache-entry-size defaults to 4KB (or
180  - The L2 cache entry size has the same restrictions as the cluster
183  - Smaller entry sizes generally improve the cache efficiency and make
194  - Only the L2 cache can be configured this way. The refcount cache
197  - If the L2 cache is big enough to hold all of the image's L2 tables
198    (as explained in the "Choosing the right cache sizes" and "How to
199    configure the cache sizes" sections in this document) then none of
200    this is necessary and you can omit the "l2-cache-entry-size"
207 It is possible to clean unused cache entries in order to reduce the
210 The parameter "cache-clean-interval" defines an interval (in seconds),
211 after which all the cache entries that haven't been accessed during the
215 The following example removes all unused cache entries every 15 minutes:
217    -drive file=hd.qcow2,cache-clean-interval=900
224 so cache-clean-interval is not supported on other systems.
233 cache size must be twice as large for the same disk space.