qemu/docs/qcow2-cache.txt

11 The QEMU qcow2 driver has two caches that can improve the I/O
45 There can be many L2 tables, depending on how much space has been
49 reading the table for each I/O operation can be expensive, QEMU keeps
52 The size of the L2 cache can be configured, and setting the right
53 value can improve the I/O performance significantly.
69 bits, but a different value can be set using the refcount_bits option:
74 aforementioned L2 cache, and its size can also be configured.
82 The part of the virtual disk that can be mapped by the L2 and refcount
114 Cache sizes can be configured using the -drive option in the
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
145    can be set simultaneously.
151 (this can change if you are using internal snapshots, so you may want
161 The qcow2 L2 cache can store complete tables. This means that if QEMU
166 This can be inefficient with large cluster sizes since it results in
169 Since QEMU 2.12 you can change the size of the L2 cache entry and make
170 it smaller than the cluster size. This can be configured using the
194  - Only the L2 cache can be configured this way. The refcount cache
200    this is necessary and you can omit the "l2-cache-entry-size"