Merge remote-tracking branch 'remotes/kraxel/tags/vga-20191220-pull-request' into...

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

diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
index c459bf5dd3b5e157da15c71b31439be8710b292d..d57f40986139ba167e2372cb90c3381606daf0c7 100644 (file)
--- a/docs/qcow2-cache.txt
+++ b/docs/qcow2-cache.txt
@@ -55,7 +55,7 @@ value can improve the I/O performance significantly.
 
 The refcount blocks
 -------------------
-The qcow2 format also mantains a reference count for each cluster.
+The qcow2 format also maintains a reference count for each cluster.
 Reference counts are used for cluster allocation and internal
 snapshots. The data is stored in a two-level structure similar to the
 L1/L2 tables described above.
@@ -158,10 +158,10 @@ refcount cache is as small as possible unless overridden by the user.
 
 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.
+The qcow2 L2 cache can store complete tables. 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.
@@ -172,6 +172,9 @@ 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
 
+Since QEMU 4.0 the value of l2-cache-entry-size defaults to 4KB (or
+the cluster size if it's smaller).
+
 Some things to take into account:
 
  - The L2 cache entry size has the same restrictions as the cluster
@@ -185,7 +188,8 @@ Some things to take into account:
 
  - 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).
+   good default (usually 4096 bytes in the case of ext4, hence the
+   default).
 
  - Only the L2 cache can be configured this way. The refcount cache
    always uses the cluster size as the entry size.
@@ -194,7 +198,8 @@ Some things to take into account:
    (as explained in the "Choosing the right cache sizes" and "How to
    configure the cache sizes" sections in this document) then none of
    this is necessary and you can omit the "l2-cache-entry-size"
-   parameter altogether.
+   parameter altogether. In this case QEMU makes the entry size
+   equal to the cluster size by default.
 
 
 Reducing the memory usage