129 lines
4.4 KiB
Plaintext
129 lines
4.4 KiB
Plaintext
XBZRLE (Xor Based Zero Run Length Encoding)
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===========================================
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Using XBZRLE (Xor Based Zero Run Length Encoding) allows for the reduction
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of VM downtime and the total live-migration time of Virtual machines.
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It is particularly useful for virtual machines running memory write intensive
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workloads that are typical of large enterprise applications such as SAP ERP
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Systems, and generally speaking for any application that uses a sparse memory
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update pattern.
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Instead of sending the changed guest memory page this solution will send a
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compressed version of the updates, thus reducing the amount of data sent during
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live migration.
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In order to be able to calculate the update, the previous memory pages need to
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be stored on the source. Those pages are stored in a dedicated cache
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(hash table) and are accessed by their address.
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The larger the cache size the better the chances are that the page has already
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been stored in the cache.
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A small cache size will result in high cache miss rate.
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Cache size can be changed before and during migration.
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Format
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=======
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The compression format performs a XOR between the previous and current content
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of the page, where zero represents an unchanged value.
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The page data delta is represented by zero and non zero runs.
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A zero run is represented by its length (in bytes).
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A non zero run is represented by its length (in bytes) and the new data.
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The run length is encoded using ULEB128 (http://en.wikipedia.org/wiki/LEB128)
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There can be more than one valid encoding, the sender may send a longer encoding
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for the benefit of reducing computation cost.
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page = zrun nzrun
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| zrun nzrun page
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zrun = length
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nzrun = length byte...
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length = uleb128 encoded integer
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On the sender side XBZRLE is used as a compact delta encoding of page updates,
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retrieving the old page content from the cache (default size of 512 MB). The
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receiving side uses the existing page's content and XBZRLE to decode the new
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page's content.
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This work was originally based on research results published
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VEE 2011: Evaluation of Delta Compression Techniques for Efficient Live
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Migration of Large Virtual Machines by Benoit, Svard, Tordsson and Elmroth.
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Additionally the delta encoder XBRLE was improved further using the XBZRLE
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instead.
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XBZRLE has a sustained bandwidth of 2-2.5 GB/s for typical workloads making it
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ideal for in-line, real-time encoding such as is needed for live-migration.
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Example
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old buffer:
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1001 zeros
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05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 68 00 00 6b 00 6d
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3074 zeros
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new buffer:
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1001 zeros
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01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 68 00 00 67 00 69
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3074 zeros
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encoded buffer:
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encoded length 24
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e9 07 0f 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 03 01 67 01 01 69
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Usage
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======================
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1. Verify the destination QEMU version is able to decode the new format.
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{qemu} info migrate_capabilities
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{qemu} xbzrle: off , ...
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2. Activate xbzrle on both source and destination:
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{qemu} migrate_set_capability xbzrle on
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3. Set the XBZRLE cache size - the cache size is in MBytes and should be a
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power of 2. The cache default value is 64MBytes. (on source only)
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{qemu} migrate_set_cache_size 256m
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4. Start outgoing migration
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{qemu} migrate -d tcp:destination.host:4444
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{qemu} info migrate
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capabilities: xbzrle: on
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Migration status: active
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transferred ram: A kbytes
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remaining ram: B kbytes
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total ram: C kbytes
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total time: D milliseconds
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duplicate: E pages
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normal: F pages
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normal bytes: G kbytes
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cache size: H bytes
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xbzrle transferred: I kbytes
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xbzrle pages: J pages
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xbzrle cache miss: K
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xbzrle overflow : L
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xbzrle cache-miss: the number of cache misses to date - high cache-miss rate
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indicates that the cache size is set too low.
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xbzrle overflow: the number of overflows in the decoding which where the delta
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could not be compressed. This can happen if the changes in the pages are too
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large or there are many short changes; for example, changing every second byte
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(half a page).
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Testing: Testing indicated that live migration with XBZRLE was completed in 110
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seconds, whereas without it would not be able to complete.
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A simple synthetic memory r/w load generator:
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.. include <stdlib.h>
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.. include <stdio.h>
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.. int main()
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.. {
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.. char *buf = (char *) calloc(4096, 4096);
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.. while (1) {
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.. int i;
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.. for (i = 0; i < 4096 * 4; i++) {
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.. buf[i * 4096 / 4]++;
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.. }
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.. printf(".");
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.. }
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.. }
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