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[Qemu-block] [PATCH] docs: document how to use the l2-cache-entry-size p
[Qemu-block] [PATCH] docs: document how to use the l2-cache-entry-size parameter
Mon, 19 Feb 2018 16:54:59 +0200
This patch updates docs/qcow2-cache.txt explaining how to use the new
Here's a more detailed technical description of this feature:
And here are some performance numbers:
Signed-off-by: Alberto Garcia <address@hidden>
docs/qcow2-cache.txt | 46 +++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 43 insertions(+), 3 deletions(-)
diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
index b0571de4b8..170191a242 100644
@@ -1,6 +1,6 @@
qcow2 L2/refcount cache configuration
-Copyright (C) 2015 Igalia, S.L.
+Copyright (C) 2015, 2018 Igalia, S.L.
Author: Alberto Garcia <address@hidden>
This work is licensed under the terms of the GNU GPL, version 2 or
@@ -118,8 +118,8 @@ There are three options available, and all of them take
There are two things that need to be taken into account:
- - Both caches must have a size that is a multiple of the cluster
+ - 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
@@ -143,6 +143,46 @@ much less often than the L2 cache, so it's perfectly
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
+ -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
- [Qemu-block] [PATCH] docs: document how to use the l2-cache-entry-size parameter,
Alberto Garcia <=