1XBZRLE (Xor Based Zero Run Length Encoding) 2=========================================== 3 4Using XBZRLE (Xor Based Zero Run Length Encoding) allows for the reduction 5of VM downtime and the total live-migration time of Virtual machines. 6It is particularly useful for virtual machines running memory write intensive 7workloads that are typical of large enterprise applications such as SAP ERP 8Systems, and generally speaking for any application that uses a sparse memory 9update pattern. 10 11Instead of sending the changed guest memory page this solution will send a 12compressed version of the updates, thus reducing the amount of data sent during 13live migration. 14In order to be able to calculate the update, the previous memory pages need to 15be stored on the source. Those pages are stored in a dedicated cache 16(hash table) and are accessed by their address. 17The larger the cache size the better the chances are that the page has already 18been stored in the cache. 19A small cache size will result in high cache miss rate. 20Cache size can be changed before and during migration. 21 22Format 23======= 24 25The compression format performs a XOR between the previous and current content 26of the page, where zero represents an unchanged value. 27The page data delta is represented by zero and non zero runs. 28A zero run is represented by its length (in bytes). 29A non zero run is represented by its length (in bytes) and the new data. 30The run length is encoded using ULEB128 (http://en.wikipedia.org/wiki/LEB128) 31 32There can be more than one valid encoding, the sender may send a longer encoding 33for the benefit of reducing computation cost. 34 35page = zrun nzrun 36 | zrun nzrun page 37 38zrun = length 39 40nzrun = length byte... 41 42length = uleb128 encoded integer 43 44On the sender side XBZRLE is used as a compact delta encoding of page updates, 45retrieving the old page content from the cache (default size of 64MB). The 46receiving side uses the existing page's content and XBZRLE to decode the new 47page's content. 48 49This work was originally based on research results published 50VEE 2011: Evaluation of Delta Compression Techniques for Efficient Live 51Migration of Large Virtual Machines by Benoit, Svard, Tordsson and Elmroth. 52Additionally the delta encoder XBRLE was improved further using the XBZRLE 53instead. 54 55XBZRLE has a sustained bandwidth of 2-2.5 GB/s for typical workloads making it 56ideal for in-line, real-time encoding such as is needed for live-migration. 57 58Example 59old buffer: 601001 zeros 6105 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 68 00 00 6b 00 6d 623074 zeros 63 64new buffer: 651001 zeros 6601 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 68 00 00 67 00 69 673074 zeros 68 69encoded buffer: 70 71encoded length 24 72e9 07 0f 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 03 01 67 01 01 69 73 74Cache update strategy 75===================== 76Keeping the hot pages in the cache is effective for decreasing cache 77misses. XBZRLE uses a counter as the age of each page. The counter will 78increase after each ram dirty bitmap sync. When a cache conflict is 79detected, XBZRLE will only evict pages in the cache that are older than 80a threshold. 81 82Usage 83====================== 841. Verify the destination QEMU version is able to decode the new format. 85 {qemu} info migrate_capabilities 86 {qemu} xbzrle: off , ... 87 882. Activate xbzrle on both source and destination: 89 {qemu} migrate_set_capability xbzrle on 90 913. Set the XBZRLE cache size - the cache size is in MBytes and should be a 92power of 2. The cache default value is 64MBytes. (on source only) 93 {qemu} migrate_set_cache_size 256m 94 95Commit 73af8dd8d7 "migration: Make xbzrle_cache_size a migration parameter" 96(v2.11.0) deprecated migrate-set-cache-size, therefore, the new parameter 97is recommended. 98 {qemu} migrate_set_parameter xbzrle-cache-size 256m 99 1004. Start outgoing migration 101 {qemu} migrate -d tcp:destination.host:4444 102 {qemu} info migrate 103 capabilities: xbzrle: on 104 Migration status: active 105 transferred ram: A kbytes 106 remaining ram: B kbytes 107 total ram: C kbytes 108 total time: D milliseconds 109 duplicate: E pages 110 normal: F pages 111 normal bytes: G kbytes 112 cache size: H bytes 113 xbzrle transferred: I kbytes 114 xbzrle pages: J pages 115 xbzrle cache miss: K pages 116 xbzrle cache miss rate: L 117 xbzrle encoding rate: M 118 xbzrle overflow: N 119 120xbzrle cache miss: the number of cache misses to date - high cache-miss rate 121indicates that the cache size is set too low. 122xbzrle overflow: the number of overflows in the decoding which where the delta 123could not be compressed. This can happen if the changes in the pages are too 124large or there are many short changes; for example, changing every second byte 125(half a page). 126 127Testing: Testing indicated that live migration with XBZRLE was completed in 110 128seconds, whereas without it would not be able to complete. 129 130A simple synthetic memory r/w load generator: 131.. include <stdlib.h> 132.. include <stdio.h> 133.. int main() 134.. { 135.. char *buf = (char *) calloc(4096, 4096); 136.. while (1) { 137.. int i; 138.. for (i = 0; i < 4096 * 4; i++) { 139.. buf[i * 4096 / 4]++; 140.. } 141.. printf("."); 142.. } 143.. } 144