qcow2-cache.txt

qcow2 L2/refcount cache configuration
=====================================
Copyright (C) 2015, 2018 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/interop/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
   (or the cache entry size: see "Using smaller cache sizes" below).

 - 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.


Using smaller cache entries
---------------------------
The qcow2 L2 cache stores complete tables by default. This means that
if QEMU needs an entry from an L2 table then the whole table is read
from disk and is kept in the cache. If the cache is full then a
complete table needs to be evicted first.

This can be inefficient with large cluster sizes since it results in
more disk I/O and wastes more cache memory.

Since QEMU 2.12 you can change the size of the L2 cache entry and make
it smaller than the cluster size. This can be configured using the
"l2-cache-entry-size" parameter:

   -drive file=hd.qcow2,l2-cache-size=2097152,l2-cache-entry-size=4096

Some things to take into account:

 - The L2 cache entry size has the same restrictions as the cluster
   size (power of two, at least 512 bytes).

 - Smaller entry sizes generally improve the cache efficiency and make
   disk I/O faster. This is particularly true with solid state drives
   so it's a good idea to reduce the entry size in those cases. With
   rotating hard drives the situation is a bit more complicated so you
   should test it first and stay with the default size if unsure.

 - Try different entry sizes to see which one gives faster performance
   in your case. The block size of the host filesystem is generally a
   good default (usually 4096 bytes in the case of ext4).

 - Only the L2 cache can be configured this way. The refcount cache
   always uses the cluster size as the entry size.

 - If the L2 cache is big enough to hold all of the image's L2 tables
   (as explained in the "Choosing the right cache sizes" section
   earlier in this document) then none of this is necessary and you
   can omit the "l2-cache-entry-size" parameter altogether.


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. This is a
Linux-specific feature, so cache-clean-interval is not supported in
other systems.