Signed-off-by: Leonid Bloch <lbl...@janustech.com>
---
docs/qcow2-cache.txt | 20 +++++++++++++-------
qemu-options.hx | 9 ++++++---
2 files changed, 19 insertions(+), 10 deletions(-)
diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
index 8a09a5cc5f..2326db01b9 100644
--- a/docs/qcow2-cache.txt
+++ b/docs/qcow2-cache.txt
@@ -77,7 +77,7 @@ 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 the allocated space.
The amount of virtual disk that can be mapped by the L2 and refcount
caches (in bytes) is:
@@ -86,7 +86,7 @@ caches (in bytes) is:
disk_size = refcount_cache_size * cluster_size * 8 / refcount_bits
With the default values for cluster_size (64KB) and refcount_bits
-(16), that is
+(16), this becomes:
disk_size = l2_cache_size * 8192
disk_size = refcount_cache_size * 32768
@@ -97,12 +97,15 @@ 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
+For example, 1MB of L2 cache is needed to cover every 8 GB of the virtual
+image size (given that the default cluster size is used):
- 1048576 / 131072 = 8 GB of virtual disk covered by that cache
- 262144 / 32768 = 8 GB
+ 8 * 131072 = 1 MB
+
+A default refcount cache is 4 times the cluster size, which defaults to
+256 KB (262144 bytes). This is sufficient for 8 GB of image size:
+
+ 262144 / 32768 = 8 GB
How to configure the cache sizes
@@ -130,6 +133,9 @@ There are a few things that need to be taken into account:
memory as possible to the L2 cache before increasing the refcount
cache size.
+ - At most two of "l2-cache-size", "refcount-cache-size", and "cache-size"
+ can be set simultaneously.
+
Unlike L2 tables, refcount blocks are not used during normal I/O but
only during allocations and internal snapshots. In most cases they are
accessed sequentially (even during random guest I/O) so increasing the
diff --git a/qemu-options.hx b/qemu-options.hx
index b1bf0f485f..f6804758d3 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -752,15 +752,18 @@ image file)
@item cache-size
The maximum total size of the L2 table and refcount block caches in bytes
-(default: 1048576 bytes or 8 clusters, whichever is larger)
+(default: the sum of l2-cache-size and refcount-cache-size)
@item l2-cache-size
The maximum size of the L2 table cache in bytes
-(default: 4/5 of the total cache size)
+(default: if cache-size is not defined - 1048576 bytes or 8 clusters, whichever
+is larger; otherwise, as large as possible or needed within the cache-size,
+while permitting the requested or the minimal refcount cache size)
@item refcount-cache-size
The maximum size of the refcount block cache in bytes
-(default: 1/5 of the total cache size)
+(default: 4 times the cluster size; or if cache-size is specified, the part of
+it which is not used for the L2 cache)
@item cache-clean-interval
Clean unused entries in the L2 and refcount caches. The interval is in seconds.
--
2.17.1
