This patch updates docs/qcow2-cache.txt explaining how to use the new l2-cache-entry-size parameter.
Here's a more detailed technical description of this feature: https://lists.gnu.org/archive/html/qemu-block/2017-09/msg00635.html And here are some performance numbers: https://lists.gnu.org/archive/html/qemu-block/2017-12/msg00507.html Signed-off-by: Alberto Garcia <be...@igalia.com> --- 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 --- a/docs/qcow2-cache.txt +++ b/docs/qcow2-cache.txt @@ -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 <be...@igalia.com> 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 bytes: 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. + - 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 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 -- 2.11.0