mirror of https://gitee.com/openkylin/qemu.git
166 lines
5.7 KiB
Plaintext
166 lines
5.7 KiB
Plaintext
qcow2 L2/refcount cache configuration
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=====================================
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Copyright (C) 2015 Igalia, S.L.
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Author: Alberto Garcia <berto@igalia.com>
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This work is licensed under the terms of the GNU GPL, version 2 or
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later. See the COPYING file in the top-level directory.
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Introduction
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------------
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The QEMU qcow2 driver has two caches that can improve the I/O
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performance significantly. However, setting the right cache sizes is
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not a straightforward operation.
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This document attempts to give an overview of the L2 and refcount
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caches, and how to configure them.
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Please refer to the docs/specs/qcow2.txt file for an in-depth
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technical description of the qcow2 file format.
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Clusters
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--------
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A qcow2 file is organized in units of constant size called clusters.
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The cluster size is configurable, but it must be a power of two and
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its value 512 bytes or higher. QEMU currently defaults to 64 KB
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clusters, and it does not support sizes larger than 2MB.
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The 'qemu-img create' command supports specifying the size using the
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cluster_size option:
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qemu-img create -f qcow2 -o cluster_size=128K hd.qcow2 4G
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The L2 tables
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-------------
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The qcow2 format uses a two-level structure to map the virtual disk as
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seen by the guest to the disk image in the host. These structures are
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called the L1 and L2 tables.
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There is one single L1 table per disk image. The table is small and is
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always kept in memory.
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There can be many L2 tables, depending on how much space has been
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allocated in the image. Each table is one cluster in size. In order to
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read or write data from the virtual disk, QEMU needs to read its
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corresponding L2 table to find out where that data is located. Since
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reading the table for each I/O operation can be expensive, QEMU keeps
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an L2 cache in memory to speed up disk access.
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The size of the L2 cache can be configured, and setting the right
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value can improve the I/O performance significantly.
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The refcount blocks
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-------------------
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The qcow2 format also mantains a reference count for each cluster.
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Reference counts are used for cluster allocation and internal
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snapshots. The data is stored in a two-level structure similar to the
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L1/L2 tables described above.
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The second level structures are called refcount blocks, are also one
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cluster in size and the number is also variable and dependent on the
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amount of allocated space.
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Each block contains a number of refcount entries. Their size (in bits)
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is a power of two and must not be higher than 64. It defaults to 16
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bits, but a different value can be set using the refcount_bits option:
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qemu-img create -f qcow2 -o refcount_bits=8 hd.qcow2 4G
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QEMU keeps a refcount cache to speed up I/O much like the
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aforementioned L2 cache, and its size can also be configured.
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Choosing the right cache sizes
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------------------------------
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In order to choose the cache sizes we need to know how they relate to
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the amount of allocated space.
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The amount of virtual disk that can be mapped by the L2 and refcount
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caches (in bytes) is:
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disk_size = l2_cache_size * cluster_size / 8
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disk_size = refcount_cache_size * cluster_size * 8 / refcount_bits
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With the default values for cluster_size (64KB) and refcount_bits
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(16), that is
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disk_size = l2_cache_size * 8192
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disk_size = refcount_cache_size * 32768
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So in order to cover n GB of disk space with the default values we
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need:
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l2_cache_size = disk_size_GB * 131072
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refcount_cache_size = disk_size_GB * 32768
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QEMU has a default L2 cache of 1MB (1048576 bytes) and a refcount
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cache of 256KB (262144 bytes), so using the formulas we've just seen
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we have
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1048576 / 131072 = 8 GB of virtual disk covered by that cache
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262144 / 32768 = 8 GB
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How to configure the cache sizes
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--------------------------------
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Cache sizes can be configured using the -drive option in the
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command-line, or the 'blockdev-add' QMP command.
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There are three options available, and all of them take bytes:
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"l2-cache-size": maximum size of the L2 table cache
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"refcount-cache-size": maximum size of the refcount block cache
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"cache-size": maximum size of both caches combined
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There are two things that need to be taken into account:
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- Both caches must have a size that is a multiple of the cluster
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size.
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- If you only set one of the options above, QEMU will automatically
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adjust the others so that the L2 cache is 4 times bigger than the
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refcount cache.
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This means that these options are equivalent:
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-drive file=hd.qcow2,l2-cache-size=2097152
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-drive file=hd.qcow2,refcount-cache-size=524288
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-drive file=hd.qcow2,cache-size=2621440
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The reason for this 1/4 ratio is to ensure that both caches cover the
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same amount of disk space. Note however that this is only valid with
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the default value of refcount_bits (16). If you are using a different
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value you might want to calculate both cache sizes yourself since QEMU
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will always use the same 1/4 ratio.
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It's also worth mentioning that there's no strict need for both caches
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to cover the same amount of disk space. The refcount cache is used
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much less often than the L2 cache, so it's perfectly reasonable to
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keep it small.
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Reducing the memory usage
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-------------------------
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It is possible to clean unused cache entries in order to reduce the
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memory usage during periods of low I/O activity.
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The parameter "cache-clean-interval" defines an interval (in seconds).
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All cache entries that haven't been accessed during that interval are
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removed from memory.
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This example removes all unused cache entries every 15 minutes:
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-drive file=hd.qcow2,cache-clean-interval=900
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If unset, the default value for this parameter is 0 and it disables
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this feature.
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Note that this functionality currently relies on the MADV_DONTNEED
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argument for madvise() to actually free the memory. This is a
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Linux-specific feature, so cache-clean-interval is not supported in
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other systems.
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