1 qcow2 L2/refcount cache configuration
2 =====================================
3 Copyright (C) 2015, 2018 Igalia, S.L.
6 This work is licensed under the terms of the GNU GPL, version 2 or
7 later. See the COPYING file in the top-level directory.
11 The QEMU qcow2 driver has two caches that can improve the I/O
12 performance significantly. However, setting the right cache sizes is
13 not a straightforward operation.
15 This document attempts to give an overview of the L2 and refcount
16 caches, and how to configure them.
18 Please refer to the docs/interop/qcow2.txt file for an in-depth
19 technical description of the qcow2 file format.
24 A qcow2 file is organized in units of constant size called clusters.
26 The cluster size is configurable, but it must be a power of two and
27 its value 512 bytes or higher. QEMU currently defaults to 64 KB
28 clusters, and it does not support sizes larger than 2MB.
30 The 'qemu-img create' command supports specifying the size using the
33 qemu-img create -f qcow2 -o cluster_size=128K hd.qcow2 4G
38 The qcow2 format uses a two-level structure to map the virtual disk as
39 seen by the guest to the disk image in the host. These structures are
40 called the L1 and L2 tables.
42 There is one single L1 table per disk image. The table is small and is
43 always kept in memory.
45 There can be many L2 tables, depending on how much space has been
46 allocated in the image. Each table is one cluster in size. In order to
47 read or write data from the virtual disk, QEMU needs to read its
48 corresponding L2 table to find out where that data is located. Since
49 reading the table for each I/O operation can be expensive, QEMU keeps
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
53 value can improve the I/O performance significantly.
58 The qcow2 format also mantains a reference count for each cluster.
59 Reference counts are used for cluster allocation and internal
60 snapshots. The data is stored in a two-level structure similar to the
61 L1/L2 tables described above.
63 The second level structures are called refcount blocks, are also one
64 cluster in size and the number is also variable and dependent on the
65 amount of allocated space.
67 Each block contains a number of refcount entries. Their size (in bits)
68 is a power of two and must not be higher than 64. It defaults to 16
69 bits, but a different value can be set using the refcount_bits option:
71 qemu-img create -f qcow2 -o refcount_bits=8 hd.qcow2 4G
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
78 ------------------------------
79 In order to choose the cache sizes we need to know how they relate to
80 the amount of allocated space.
82 The part of the virtual disk that can be mapped by the L2 and refcount
85 disk_size = l2_cache_size * cluster_size / 8
86 disk_size = refcount_cache_size * cluster_size * 8 / refcount_bits
88 With the default values for cluster_size (64KB) and refcount_bits
91 disk_size = l2_cache_size * 8192
92 disk_size = refcount_cache_size * 32768
94 So in order to cover n GB of disk space with the default values we
97 l2_cache_size = disk_size_GB * 131072
98 refcount_cache_size = disk_size_GB * 32768
100 For example, 1MB of L2 cache is needed to cover every 8 GB of the virtual
101 image size (given that the default cluster size is used):
105 The refcount cache is 4 times the cluster size by default. With the default
106 cluster size of 64 KB, it is 256 KB (262144 bytes). This is sufficient for
109 262144 * 32768 = 8 GB
112 How to configure the cache sizes
113 --------------------------------
114 Cache sizes can be configured using the -drive option in the
115 command-line, or the 'blockdev-add' QMP command.
117 There are three options available, and all of them take bytes:
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
123 There are a few things that need to be taken into account:
125 - Both caches must have a size that is a multiple of the cluster size
126 (or the cache entry size: see "Using smaller cache sizes" below).
128 - The maximum L2 cache size is 1 MB by default (enough for full coverage
129 of 8 GB images, with the default cluster size). This value can be
130 modified using the "l2-cache-size" option. QEMU will not use more memory
131 than needed to hold all of the image's L2 tables, regardless of this max.
132 value. The minimal L2 cache size is 2 clusters (or 2 cache entries, see
135 - The default (and minimum) refcount cache size is 4 clusters.
137 - If only "cache-size" is specified then QEMU will assign as much
138 memory as possible to the L2 cache before increasing the refcount
141 - At most two of "l2-cache-size", "refcount-cache-size", and "cache-size"
142 can be set simultaneously.
144 Unlike L2 tables, refcount blocks are not used during normal I/O but
145 only during allocations and internal snapshots. In most cases they are
146 accessed sequentially (even during random guest I/O) so increasing the
147 refcount cache size won't have any measurable effect in performance
148 (this can change if you are using internal snapshots, so you may want
149 to think about increasing the cache size if you use them heavily).
151 Before QEMU 2.12 the refcount cache had a default size of 1/4 of the
152 L2 cache size. This resulted in unnecessarily large caches, so now the
153 refcount cache is as small as possible unless overridden by the user.
156 Using smaller cache entries
157 ---------------------------
158 The qcow2 L2 cache stores complete tables by default. This means that
159 if QEMU needs an entry from an L2 table then the whole table is read
160 from disk and is kept in the cache. If the cache is full then a
161 complete table needs to be evicted first.
163 This can be inefficient with large cluster sizes since it results in
164 more disk I/O and wastes more cache memory.
166 Since QEMU 2.12 you can change the size of the L2 cache entry and make
167 it smaller than the cluster size. This can be configured using the
168 "l2-cache-entry-size" parameter:
170 -drive file=hd.qcow2,l2-cache-size=2097152,l2-cache-entry-size=4096
172 Some things to take into account:
174 - The L2 cache entry size has the same restrictions as the cluster
175 size (power of two, at least 512 bytes).
177 - Smaller entry sizes generally improve the cache efficiency and make
178 disk I/O faster. This is particularly true with solid state drives
179 so it's a good idea to reduce the entry size in those cases. With
180 rotating hard drives the situation is a bit more complicated so you
181 should test it first and stay with the default size if unsure.
183 - Try different entry sizes to see which one gives faster performance
184 in your case. The block size of the host filesystem is generally a
185 good default (usually 4096 bytes in the case of ext4).
187 - Only the L2 cache can be configured this way. The refcount cache
188 always uses the cluster size as the entry size.
190 - If the L2 cache is big enough to hold all of the image's L2 tables
191 (as explained in the "Choosing the right cache sizes" and "How to
192 configure the cache sizes" sections in this document) then none of
193 this is necessary and you can omit the "l2-cache-entry-size"
194 parameter altogether.
197 Reducing the memory usage
198 -------------------------
199 It is possible to clean unused cache entries in order to reduce the
200 memory usage during periods of low I/O activity.
202 The parameter "cache-clean-interval" defines an interval (in seconds).
203 All cache entries that haven't been accessed during that interval are
206 This example removes all unused cache entries every 15 minutes:
208 -drive file=hd.qcow2,cache-clean-interval=900
210 If unset, the default value for this parameter is 0 and it disables
213 Note that this functionality currently relies on the MADV_DONTNEED
214 argument for madvise() to actually free the memory. This is a
215 Linux-specific feature, so cache-clean-interval is not supported in