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Re: [Qemu-devel] [PATCH 8/8] docs: document how to configure the qcow2 L

From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH 8/8] docs: document how to configure the qcow2 L2/refcount caches
Date: Mon, 11 May 2015 15:23:25 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0

On 11.05.2015 14:55, Alberto Garcia wrote:
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.

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

Signed-off-by: Alberto Garcia <address@hidden>
  docs/qcow2-cache.txt | 117 +++++++++++++++++++++++++++++++++++++++++++++++++++
  1 file changed, 117 insertions(+)
  create mode 100644 docs/qcow2-cache.txt

diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
new file mode 100644
index 0000000..2570172
--- /dev/null
+++ b/docs/qcow2-cache.txt
@@ -0,0 +1,117 @@
+qcow2 L2/refcount cache configuration
+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.
+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.img 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. 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.
+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 / 2

Only with the default of refcount_bits=16. In the general case, it's refcount_cache_size * cluster_size * 8 / refcount_bits.

+With the default cluster size (64KB), 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 cluster size
+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.
+ - The L2 cache should be 4 times bigger than the refcount cache in
+   order to cover the same amount of disk space.

Again, only for refcount_bits=16. For the general case, it's "8 * 8 / refcount_bits = 64 / refcount_bits times as big".

+However, if you only set one of the options above, QEMU will
+automatically adjust the others so that the 1/4 ratio is maintained.
+This means that these options are equivalent:
+-drive file=hd.img,l2-cache-size=2097152
+-drive file=hd.img,refcount-cache-size=524288
+-drive file=hd.img,cache-size=2621440

Apart from this minor thing, looks good. Well, except that maybe it should be preferred to use .qcow2 as the extension for qcow2 files (instead of .img).


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