qemu/docs/qcow2-cache.txt

1 qcow2 L2/refcount cache configuration
15 This document attempts to give an overview of the L2 and refcount
36 The L2 tables
40 called the L1 and L2 tables.
45 There can be many L2 tables, depending on how much space has been
48 corresponding L2 table to find out where that data is located. Since
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
61 L1/L2 tables described above.
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
100 For example, 1MB of L2 cache is needed to cover every 8 GB of the virtual
119 "l2-cache-size":         maximum size of the L2 table cache
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
131    more memory than needed to hold all of the image's L2 tables, regardless
136    The minimal L2 cache size is 2 clusters (or 2 cache entries, see below).
141    memory as possible to the L2 cache before increasing the refcount
144  - At most two of "l2-cache-size", "refcount-cache-size", and "cache-size"
147 Unlike L2 tables, refcount blocks are not used during normal I/O but
155 L2 cache size. This resulted in unnecessarily large caches, so now the
161 The qcow2 L2 cache can store complete tables. This means that if QEMU
162 needs an entry from an L2 table then the whole table is read from disk
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
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
200    this is necessary and you can omit the "l2-cache-entry-size"
227 Extended L2 Entries
230 64-bit L2 entries.
232 Images with extended L2 entries need twice as much L2 metadata, so the L2