Merge remote-tracking branch 'remotes/pmaydell/tags/pull-target-arm-20181213' into...

[qemu.git] / docs / qcow2-cache.txt

diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
index 750447ea4fa31107c47cc115d351feb282fd8151..c459bf5dd3b5e157da15c71b31439be8710b292d 100644 (file)
--- a/docs/qcow2-cache.txt
+++ b/docs/qcow2-cache.txt
@@ -125,12 +125,15 @@ There are a few 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).
 
- - The maximum L2 cache size is 1 MB by default (enough for full coverage
-   of 8 GB images, with the default cluster size). This value can be
-   modified using the "l2-cache-size" option. QEMU will not use more memory
-   than needed to hold all of the image's L2 tables, regardless of this max.
-   value. The minimal L2 cache size is 2 clusters (or 2 cache entries, see
-   below).
+ - The maximum L2 cache size is 32 MB by default on Linux platforms (enough
+   for full coverage of 256 GB images, with the default cluster size). This
+   value can be modified using the "l2-cache-size" option. QEMU will not use
+   more memory than needed to hold all of the image's L2 tables, regardless
+   of this max. value.
+   On non-Linux platforms the maximal value is smaller by default (8 MB) and
+   this difference stems from the fact that on Linux the cache can be cleared
+   periodically if needed, using the "cache-clean-interval" option (see below).
+   The minimal L2 cache size is 2 clusters (or 2 cache entries, see below).
 
  - The default (and minimum) refcount cache size is 4 clusters.
 
@@ -199,18 +202,18 @@ 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.
+The parameter "cache-clean-interval" defines an interval (in seconds),
+after which all the cache entries that haven't been accessed during the
+interval are removed from memory. Setting this parameter to 0 disables this
+feature.
 
-This example removes all unused cache entries every 15 minutes:
+The following 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.
+If unset, the default value for this parameter is 600 on platforms which
+support this functionality, and is 0 (disabled) on other platforms.
 
-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.
+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 on other systems.