Commit 271e2a38a2 for qemu.org
commit 271e2a38a22c4db76e30c9293eb7f9b8b6081cd6
Author: Thomas Huth <thuth@redhat.com>
Date: Tue Mar 10 10:28:22 2026 +0100
docs: Move xbzrle.txt into the migration folder and convert to rst
xbzrle is a feature of migration and thus this file should go
into the docs/devel/migration/ folder. While we're at it, turn
it into proper .rst format, too.
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
diff --git a/docs/devel/migration/features.rst b/docs/devel/migration/features.rst
index 8f431d52f9..9aef79e7fa 100644
--- a/docs/devel/migration/features.rst
+++ b/docs/devel/migration/features.rst
@@ -15,3 +15,4 @@ Migration has plenty of features to support different use cases.
qpl-compression
uadk-compression
qatzip-compression
+ xbzrle
diff --git a/docs/xbzrle.txt b/docs/devel/migration/xbzrle.rst
similarity index 74%
rename from docs/xbzrle.txt
rename to docs/devel/migration/xbzrle.rst
index bcb3f0c901..d4f4007261 100644
--- a/docs/xbzrle.txt
+++ b/docs/devel/migration/xbzrle.rst
@@ -20,7 +20,7 @@ A small cache size will result in high cache miss rate.
Cache size can be changed before and during migration.
Format
-=======
+------
The compression format performs a XOR between the previous and current content
of the page, where zero represents an unchanged value.
@@ -29,17 +29,19 @@ A zero run is represented by its length (in bytes).
A non zero run is represented by its length (in bytes) and the new data.
The run length is encoded using ULEB128 (http://en.wikipedia.org/wiki/LEB128)
-There can be more than one valid encoding, the sender may send a longer encoding
-for the benefit of reducing computation cost.
+There can be more than one valid encoding, the sender may send a longer
+encoding for the benefit of reducing computation cost.
-page = zrun nzrun
- | zrun nzrun page
+::
-zrun = length
+ page = zrun nzrun
+ | zrun nzrun page
-nzrun = length byte...
+ zrun = length
-length = uleb128 encoded integer
+ nzrun = length byte...
+
+ length = uleb128 encoded integer
On the sender side XBZRLE is used as a compact delta encoding of page updates,
retrieving the old page content from the cache (default size of 64MB). The
@@ -55,24 +57,34 @@ instead.
XBZRLE has a sustained bandwidth of 2-2.5 GB/s for typical workloads making it
ideal for in-line, real-time encoding such as is needed for live-migration.
-Example
+Example:
+
old buffer:
-1001 zeros
-05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 68 00 00 6b 00 6d
-3074 zeros
+
+.. code:: batch
+
+ 1001 zeros
+ 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 68 00 00 6b 00 6d
+ 3074 zeros
new buffer:
-1001 zeros
-01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 68 00 00 67 00 69
-3074 zeros
+
+.. code:: batch
+
+ 1001 zeros
+ 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 68 00 00 67 00 69
+ 3074 zeros
encoded buffer:
-encoded length 24
-e9 07 0f 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 03 01 67 01 01 69
+.. code:: batch
+
+ encoded length 24
+ e9 07 0f 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 03 01 67 01 01 69
Cache update strategy
-=====================
+---------------------
+
Keeping the hot pages in the cache is effective for decreasing cache
misses. XBZRLE uses a counter as the age of each page. The counter will
increase after each ram dirty bitmap sync. When a cache conflict is
@@ -80,21 +92,27 @@ detected, XBZRLE will only evict pages in the cache that are older than
a threshold.
Usage
-======================
-1. Verify the destination QEMU version is able to decode the new format.
- {qemu} info migrate_capabilities
- {qemu} xbzrle: off , ...
+-----
-2. Activate xbzrle on both source and destination:
- {qemu} migrate_set_capability xbzrle on
+1. Verify the destination QEMU version is able to decode the new format::
+
+ (qemu) info migrate_capabilities
+ xbzrle: off
+ ...
+
+2. Activate xbzrle on both source and destination::
+
+ (qemu) migrate_set_capability xbzrle on
3. Set the XBZRLE cache size - the cache size is in MBytes and should be a
-power of 2. The cache default value is 64MBytes. (on source only)
- {qemu} migrate_set_parameter xbzrle-cache-size 256m
+ power of 2. The cache default value is 64 MBytes (on source only)::
+
+ (qemu) migrate_set_parameter xbzrle-cache-size 256m
-4. Start outgoing migration
- {qemu} migrate -d tcp:destination.host:4444
- {qemu} info migrate
+4. Start outgoing migration::
+
+ (qemu) migrate -d tcp:destination.host:4444
+ (qemu) info migrate
capabilities: xbzrle: on
Migration status: active
transferred ram: A kbytes
@@ -114,6 +132,7 @@ power of 2. The cache default value is 64MBytes. (on source only)
xbzrle cache miss: the number of cache misses to date - high cache-miss rate
indicates that the cache size is set too low.
+
xbzrle overflow: the number of overflows in the decoding which where the delta
could not be compressed. This can happen if the changes in the pages are too
large or there are many short changes; for example, changing every second byte
@@ -123,16 +142,19 @@ Testing: Testing indicated that live migration with XBZRLE was completed in 110
seconds, whereas without it would not be able to complete.
A simple synthetic memory r/w load generator:
-.. include <stdlib.h>
-.. include <stdio.h>
-.. int main()
-.. {
-.. char *buf = (char *) calloc(4096, 4096);
-.. while (1) {
-.. int i;
-.. for (i = 0; i < 4096 * 4; i++) {
-.. buf[i * 4096 / 4]++;
-.. }
-.. printf(".");
-.. }
-.. }
+
+.. code-block:: c
+
+ #include <stdlib.h>
+ #include <stdio.h>
+ int main()
+ {
+ char *buf = (char *) calloc(4096, 4096);
+ while (1) {
+ int i;
+ for (i = 0; i < 4096 * 4; i++) {
+ buf[i * 4096 / 4]++;
+ }
+ printf(".");
+ }
+ }