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[Qemu-devel] [PATCH v11 3/9] Add XBZRLE documentation
From: |
Orit Wasserman |
Subject: |
[Qemu-devel] [PATCH v11 3/9] Add XBZRLE documentation |
Date: |
Tue, 22 May 2012 15:56:59 +0300 |
Signed-off-by: Orit Wasserman <address@hidden>
---
docs/xbzrle.txt | 114 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 files changed, 114 insertions(+), 0 deletions(-)
create mode 100644 docs/xbzrle.txt
diff --git a/docs/xbzrle.txt b/docs/xbzrle.txt
new file mode 100644
index 0000000..16950d5
--- /dev/null
+++ b/docs/xbzrle.txt
@@ -0,0 +1,114 @@
+XBZRLE (Xor Based Zero Run Length Encoding)
+===========================================
+
+Using XBZRLE (Xor Based Zero Run Length Encoding) allows for the reduction of
VM
+downtime and the total live-migration time of Virtual machines.
+It is particularly useful for virtual machines running memory write intensive
+workloads that are typical of large enterprise applications such as SAP ERP
+Systems, and generally speaking for any application that uses a sparse memory
+update pattern.
+
+Instead of sending the changed guest memory page this solution will send a
+compressed version of the updates, thus reducing the amount of data sent during
+live migration.
+In order to be able to calculate the update, the previous memory pages needed
to
+be stored. Those pages are stored in a dedicated cache (hash table) and are
+accessed by their address.
+The larger the cache size the better the chances are that the page has already
+been stored in the cache.
+A small cache size will result in high cache miss rate.
+Cache size can be changed before and during migration.
+
+Format
+=======
+
+The compression format uses the zero value, where zero represents an unchanged
+value.
+The page data delta is represented by zero and non zero runs.
+A zero run is represented by it's length (in bytes).
+A non zero run is represented by it's length (in bytes) and the data.
+The run length is encoded using ULEB128 (http://en.wikipedia.org/wiki/LEB128)
+
+page = zrun nzrun
+ | zrun nzrun page
+
+zrun = length
+
+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 512 MB). The
+receiving side uses the existing page's content and XBZRLE to decode the new
+page's content.
+
+This is a more compact way to store the deltas than the previous version.
+
+This work was originally based on research results published
+VEE 2011: Evaluation of Delta Compression Techniques for Efficient Live
+Migration of Large Virtual Machines by Benoit, Svard, Tordsson and Elmroth.
+Additionally the delta encoder XBRLE was improved further using the XBZRLE
+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.
+
+Migration Capabilities
+======================
+In order to use XBZRLE the destination QEMU version should be able to
+decode the new format.
+Adding a new migration capabilities command that will allow external management
+to query for it support.
+A typical use for the destination
+ {qemu} info migrate_capabilities
+ {qemu} xbzrle, ...
+
+In order to enable capabilities for future live migration,
+a new command migrate_set_parameter is introduced:
+ {qemu} migrate_set_parameter xbzrle
+
+Usage
+======
+
+1. Activate xbzrle
+2. Set the XBZRLE cache size - the cache size is in MBytes and should be a
+power of 2. The cache default value is 64MBytes.
+3. start outgoing migration
+
+A typical usage scenario:
+ {qemu} migrate_set_parameter xbzrle
+ {qemu} migrate_set_cachesize 256m
+ {qemu} migrate -d tcp:destination.host:4444
+ {qemu} info migrate
+ ...
+ transferred ram-duplicate: A kbytes
+ transferred ram-normal: B kbytes
+ transferred ram-xbrle: C kbytes
+ overflow ram-xbrle: D pages
+ cache-miss ram-xbrle: E pages
+
+cache-miss: the number of cache misses to date - high cache-miss rate
+indicates that the cache size is set too low.
+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 change every second byte (half a
+page).
+
+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(".");
+.. }
+.. }
--
1.7.7.6
- [Qemu-devel] [PATCH v11 0/9] XBZRLE delta for live migration of large memory app, Orit Wasserman, 2012/05/22
- [Qemu-devel] [PATCH v11 1/9] Add MigrationParams structure, Orit Wasserman, 2012/05/22
- [Qemu-devel] [PATCH v11 3/9] Add XBZRLE documentation,
Orit Wasserman <=
- [Qemu-devel] [PATCH v11 6/9] Add save_block_hdr function, Orit Wasserman, 2012/05/22
- [Qemu-devel] [PATCH v11 5/9] Add uleb encoding/decoding functions, Orit Wasserman, 2012/05/22
- [Qemu-devel] [PATCH v11 7/9] Add XBZRLE to ram_save_block and ram_save_live, Orit Wasserman, 2012/05/22
- [Qemu-devel] [PATCH v11 2/9] Add migration capabilites, Orit Wasserman, 2012/05/22
- [Qemu-devel] [PATCH v11 8/9] Add set_cachesize command, Orit Wasserman, 2012/05/22
- [Qemu-devel] [PATCH v11 4/9] Add cache handling functions, Orit Wasserman, 2012/05/22
- [Qemu-devel] [PATCH v11 9/9] Add XBZRLE statistics, Orit Wasserman, 2012/05/22