205 lines
7.6 KiB
Plaintext
205 lines
7.6 KiB
Plaintext
qcow2 L2/refcount cache configuration
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=====================================
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Copyright (C) 2015, 2018 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/interop/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 a few 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 size
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(or the cache entry size: see "Using smaller cache sizes" below).
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- The default L2 cache size is 8 clusters or 1MB (whichever is more),
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and the minimum is 2 clusters (or 2 cache entries, see below).
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- The default (and minimum) refcount cache size is 4 clusters.
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- If only "cache-size" is specified then QEMU will assign as much
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memory as possible to the L2 cache before increasing the refcount
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cache size.
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Unlike L2 tables, refcount blocks are not used during normal I/O but
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only during allocations and internal snapshots. In most cases they are
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accessed sequentially (even during random guest I/O) so increasing the
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refcount cache size won't have any measurable effect in performance
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(this can change if you are using internal snapshots, so you may want
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to think about increasing the cache size if you use them heavily).
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Before QEMU 2.12 the refcount cache had a default size of 1/4 of the
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L2 cache size. This resulted in unnecessarily large caches, so now the
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refcount cache is as small as possible unless overridden by the user.
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Using smaller cache entries
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---------------------------
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The qcow2 L2 cache stores complete tables by default. This means that
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if QEMU needs an entry from an L2 table then the whole table is read
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from disk and is kept in the cache. If the cache is full then a
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complete table needs to be evicted first.
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This can be inefficient with large cluster sizes since it results in
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more disk I/O and wastes more cache memory.
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Since QEMU 2.12 you can change the size of the L2 cache entry and make
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it smaller than the cluster size. This can be configured using the
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"l2-cache-entry-size" parameter:
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-drive file=hd.qcow2,l2-cache-size=2097152,l2-cache-entry-size=4096
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Some things to take into account:
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- The L2 cache entry size has the same restrictions as the cluster
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size (power of two, at least 512 bytes).
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- Smaller entry sizes generally improve the cache efficiency and make
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disk I/O faster. This is particularly true with solid state drives
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so it's a good idea to reduce the entry size in those cases. With
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rotating hard drives the situation is a bit more complicated so you
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should test it first and stay with the default size if unsure.
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- Try different entry sizes to see which one gives faster performance
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in your case. The block size of the host filesystem is generally a
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good default (usually 4096 bytes in the case of ext4).
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- Only the L2 cache can be configured this way. The refcount cache
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always uses the cluster size as the entry size.
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- If the L2 cache is big enough to hold all of the image's L2 tables
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(as explained in the "Choosing the right cache sizes" section
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earlier in this document) then none of this is necessary and you
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can omit the "l2-cache-entry-size" parameter altogether.
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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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