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 <address@hidden>
Cc: Max Reitz <address@hidden>
---
docs/qcow2-cache.txt | 163 +++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 163 insertions(+)
create mode 100644 docs/qcow2-cache.txt
diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
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+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)
+after which all unused cache entries 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.