docs: document how to configure the qcow2 L2/refcount caches

QEMU has options to configure the size of the L2 and refcount
caches for the qcow2 format. However, choosing the right sizes for
a particular disk image is not a straightforward operation since
the ratio between the cache size and the allocated disk space is
not obvious and depends on the size of the cluster and the refcount
entries.

This document attempts to give an overview of both caches and how to
configure their sizes.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 55de928e139b1ba3f3d40fe9c6c88f30b1f36410.1438690126.git.berto@igalia.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

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