docs/qcow2-cache.txt

   1 qcow2 L2/refcount cache configuration
   2 =====================================
   3 Copyright (C) 2015, 2018 Igalia, S.L.
   4 Author: Alberto Garcia <berto@igalia.com>
   5
   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.
   8
   9 Introduction
  10 ------------
  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.
  14
  15 This document attempts to give an overview of the L2 and refcount
  16 caches, and how to configure them.
  17
  18 Please refer to the docs/interop/qcow2.txt file for an in-depth
  19 technical description of the qcow2 file format.
  20
  21
  22 Clusters
  23 --------
  24 A qcow2 file is organized in units of constant size called clusters.
  25
  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.
  29
  30 The 'qemu-img create' command supports specifying the size using the
  31 cluster_size option:
  32
  33    qemu-img create -f qcow2 -o cluster_size=128K hd.qcow2 4G
  34
  35
  36 The L2 tables
  37 -------------
  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.
  41
  42 There is one single L1 table per disk image. The table is small and is
  43 always kept in memory.
  44
  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.
  51
  52 The size of the L2 cache can be configured, and setting the right
  53 value can improve the I/O performance significantly.
  54
  55
  56 The refcount blocks
  57 -------------------
  58 The qcow2 format also maintains 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.
  62
  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.
  66
  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:
  70
  71    qemu-img create -f qcow2 -o refcount_bits=8 hd.qcow2 4G
  72
  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.
  75
  76
  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.
  81
  82 The part of the virtual disk that can be mapped by the L2 and refcount
  83 caches (in bytes) is:
  84
  85    disk_size = l2_cache_size * cluster_size / 8
  86    disk_size = refcount_cache_size * cluster_size * 8 / refcount_bits
  87
  88 With the default values for cluster_size (64KB) and refcount_bits
  89 (16), this becomes:
  90
  91    disk_size = l2_cache_size * 8192
  92    disk_size = refcount_cache_size * 32768
  93
  94 So in order to cover n GB of disk space with the default values we
  95 need:
  96
  97    l2_cache_size = disk_size_GB * 131072
  98    refcount_cache_size = disk_size_GB * 32768
  99
 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):
 102
 103    8 GB / 8192 = 1 MB
 104
 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
 107 8 GB of image size:
 108
 109    262144 * 32768 = 8 GB
 110
 111
 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.
 116
 117 There are three options available, and all of them take bytes:
 118
 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
 122
 123 There are a few things that need to be taken into account:
 124
 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).
 127
 128  - The maximum L2 cache size is 32 MB by default on Linux platforms (enough
 129    for full coverage of 256 GB images, with the default cluster size). This
 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
 132    of this max. value.
 133    On non-Linux platforms the maximal value is smaller by default (8 MB) and
 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).
 137
 138  - The default (and minimum) refcount cache size is 4 clusters.
 139
 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.
 143
 144  - At most two of "l2-cache-size", "refcount-cache-size", and "cache-size"
 145    can be set simultaneously.
 146
 147 Unlike L2 tables, refcount blocks are not used during normal I/O but
 148 only during allocations and internal snapshots. In most cases they are
 149 accessed sequentially (even during random guest I/O) so increasing the
 150 refcount cache size won't have any measurable effect in performance
 151 (this can change if you are using internal snapshots, so you may want
 152 to think about increasing the cache size if you use them heavily).
 153
 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.
 157
 158
 159 Using smaller cache entries
 160 ---------------------------
 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
 163 and is kept in the cache. If the cache is full then a complete table
 164 needs to be evicted first.
 165
 166 This can be inefficient with large cluster sizes since it results in
 167 more disk I/O and wastes more cache memory.
 168
 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
 171 "l2-cache-entry-size" parameter:
 172
 173    -drive file=hd.qcow2,l2-cache-size=2097152,l2-cache-entry-size=4096
 174
 175 Since QEMU 4.0 the value of l2-cache-entry-size defaults to 4KB (or
 176 the cluster size if it's smaller).
 177
 178 Some things to take into account:
 179
 180  - The L2 cache entry size has the same restrictions as the cluster
 181    size (power of two, at least 512 bytes).
 182
 183  - Smaller entry sizes generally improve the cache efficiency and make
 184    disk I/O faster. This is particularly true with solid state drives
 185    so it's a good idea to reduce the entry size in those cases. With
 186    rotating hard drives the situation is a bit more complicated so you
 187    should test it first and stay with the default size if unsure.
 188
 189  - Try different entry sizes to see which one gives faster performance
 190    in your case. The block size of the host filesystem is generally a
 191    good default (usually 4096 bytes in the case of ext4, hence the
 192    default).
 193
 194  - Only the L2 cache can be configured this way. The refcount cache
 195    always uses the cluster size as the entry size.
 196
 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"
 201    parameter altogether. In this case QEMU makes the entry size
 202    equal to the cluster size by default.
 203
 204
 205 Reducing the memory usage
 206 -------------------------
 207 It is possible to clean unused cache entries in order to reduce the
 208 memory usage during periods of low I/O activity.
 209
 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
 212 interval are removed from memory. Setting this parameter to 0 disables this
 213 feature.
 214
 215 The following example removes all unused cache entries every 15 minutes:
 216
 217    -drive file=hd.qcow2,cache-clean-interval=900
 218
 219 If unset, the default value for this parameter is 600 on platforms which
 220 support this functionality, and is 0 (disabled) on other platforms.
 221
 222 This functionality currently relies on the MADV_DONTNEED argument for
 223 madvise() to actually free the memory. This is a Linux-specific feature,
 224 so cache-clean-interval is not supported on other systems.