1qcow2 L2/refcount cache configuration 2===================================== 3Copyright (C) 2015, 2018 Igalia, S.L. 4Author: Alberto Garcia <berto@igalia.com> 5 6This work is licensed under the terms of the GNU GPL, version 2 or 7later. See the COPYING file in the top-level directory. 8 9Introduction 10------------ 11The QEMU qcow2 driver has two caches that can improve the I/O 12performance significantly. However, setting the right cache sizes is 13not a straightforward operation. 14 15This document attempts to give an overview of the L2 and refcount 16caches, and how to configure them. 17 18Please refer to the docs/interop/qcow2.txt file for an in-depth 19technical description of the qcow2 file format. 20 21 22Clusters 23-------- 24A qcow2 file is organized in units of constant size called clusters. 25 26The cluster size is configurable, but it must be a power of two and 27its value 512 bytes or higher. QEMU currently defaults to 64 KB 28clusters, and it does not support sizes larger than 2MB. 29 30The 'qemu-img create' command supports specifying the size using the 31cluster_size option: 32 33 qemu-img create -f qcow2 -o cluster_size=128K hd.qcow2 4G 34 35 36The L2 tables 37------------- 38The qcow2 format uses a two-level structure to map the virtual disk as 39seen by the guest to the disk image in the host. These structures are 40called the L1 and L2 tables. 41 42There is one single L1 table per disk image. The table is small and is 43always kept in memory. 44 45There can be many L2 tables, depending on how much space has been 46allocated in the image. Each table is one cluster in size. In order to 47read or write data from the virtual disk, QEMU needs to read its 48corresponding L2 table to find out where that data is located. Since 49reading the table for each I/O operation can be expensive, QEMU keeps 50an L2 cache in memory to speed up disk access. 51 52The size of the L2 cache can be configured, and setting the right 53value can improve the I/O performance significantly. 54 55 56The refcount blocks 57------------------- 58The qcow2 format also mantains a reference count for each cluster. 59Reference counts are used for cluster allocation and internal 60snapshots. The data is stored in a two-level structure similar to the 61L1/L2 tables described above. 62 63The second level structures are called refcount blocks, are also one 64cluster in size and the number is also variable and dependent on the 65amount of allocated space. 66 67Each block contains a number of refcount entries. Their size (in bits) 68is a power of two and must not be higher than 64. It defaults to 16 69bits, 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 73QEMU keeps a refcount cache to speed up I/O much like the 74aforementioned L2 cache, and its size can also be configured. 75 76 77Choosing the right cache sizes 78------------------------------ 79In order to choose the cache sizes we need to know how they relate to 80the amount of allocated space. 81 82The amount of virtual disk that can be mapped by the L2 and refcount 83caches (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 88With the default values for cluster_size (64KB) and refcount_bits 89(16), that is 90 91 disk_size = l2_cache_size * 8192 92 disk_size = refcount_cache_size * 32768 93 94So in order to cover n GB of disk space with the default values we 95need: 96 97 l2_cache_size = disk_size_GB * 131072 98 refcount_cache_size = disk_size_GB * 32768 99 100QEMU has a default L2 cache of 1MB (1048576 bytes) and a refcount 101cache of 256KB (262144 bytes), so using the formulas we've just seen 102we have 103 104 1048576 / 131072 = 8 GB of virtual disk covered by that cache 105 262144 / 32768 = 8 GB 106 107 108How to configure the cache sizes 109-------------------------------- 110Cache sizes can be configured using the -drive option in the 111command-line, or the 'blockdev-add' QMP command. 112 113There are three options available, and all of them take bytes: 114 115"l2-cache-size": maximum size of the L2 table cache 116"refcount-cache-size": maximum size of the refcount block cache 117"cache-size": maximum size of both caches combined 118 119There are a few things that need to be taken into account: 120 121 - Both caches must have a size that is a multiple of the cluster size 122 (or the cache entry size: see "Using smaller cache sizes" below). 123 124 - The default L2 cache size is 8 clusters or 1MB (whichever is more), 125 and the minimum is 2 clusters (or 2 cache entries, see below). 126 127 - The default (and minimum) refcount cache size is 4 clusters. 128 129 - If only "cache-size" is specified then QEMU will assign as much 130 memory as possible to the L2 cache before increasing the refcount 131 cache size. 132 133Unlike L2 tables, refcount blocks are not used during normal I/O but 134only during allocations and internal snapshots. In most cases they are 135accessed sequentially (even during random guest I/O) so increasing the 136refcount cache size won't have any measurable effect in performance 137(this can change if you are using internal snapshots, so you may want 138to think about increasing the cache size if you use them heavily). 139 140Before QEMU 2.12 the refcount cache had a default size of 1/4 of the 141L2 cache size. This resulted in unnecessarily large caches, so now the 142refcount cache is as small as possible unless overridden by the user. 143 144 145Using smaller cache entries 146--------------------------- 147The qcow2 L2 cache stores complete tables by default. This means that 148if QEMU needs an entry from an L2 table then the whole table is read 149from disk and is kept in the cache. If the cache is full then a 150complete table needs to be evicted first. 151 152This can be inefficient with large cluster sizes since it results in 153more disk I/O and wastes more cache memory. 154 155Since QEMU 2.12 you can change the size of the L2 cache entry and make 156it smaller than the cluster size. This can be configured using the 157"l2-cache-entry-size" parameter: 158 159 -drive file=hd.qcow2,l2-cache-size=2097152,l2-cache-entry-size=4096 160 161Some things to take into account: 162 163 - The L2 cache entry size has the same restrictions as the cluster 164 size (power of two, at least 512 bytes). 165 166 - Smaller entry sizes generally improve the cache efficiency and make 167 disk I/O faster. This is particularly true with solid state drives 168 so it's a good idea to reduce the entry size in those cases. With 169 rotating hard drives the situation is a bit more complicated so you 170 should test it first and stay with the default size if unsure. 171 172 - Try different entry sizes to see which one gives faster performance 173 in your case. The block size of the host filesystem is generally a 174 good default (usually 4096 bytes in the case of ext4). 175 176 - Only the L2 cache can be configured this way. The refcount cache 177 always uses the cluster size as the entry size. 178 179 - If the L2 cache is big enough to hold all of the image's L2 tables 180 (as explained in the "Choosing the right cache sizes" section 181 earlier in this document) then none of this is necessary and you 182 can omit the "l2-cache-entry-size" parameter altogether. 183 184 185Reducing the memory usage 186------------------------- 187It is possible to clean unused cache entries in order to reduce the 188memory usage during periods of low I/O activity. 189 190The parameter "cache-clean-interval" defines an interval (in seconds). 191All cache entries that haven't been accessed during that interval are 192removed from memory. 193 194This example removes all unused cache entries every 15 minutes: 195 196 -drive file=hd.qcow2,cache-clean-interval=900 197 198If unset, the default value for this parameter is 0 and it disables 199this feature. 200 201Note that this functionality currently relies on the MADV_DONTNEED 202argument for madvise() to actually free the memory. This is a 203Linux-specific feature, so cache-clean-interval is not supported in 204other systems. 205