# 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 need to be stored on the source. 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 performs a XOR between the previous and current content of the page, where zero represents an unchanged value. The page data delta is represented by zero and non zero runs. 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.

    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 64MB). The receiving side uses the existing page’s content and XBZRLE to decode the new page’s content.

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.

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

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

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

## 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 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
       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 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
       capabilities: xbzrle: on
       Migration status: active
       transferred ram: A kbytes
       remaining ram: B kbytes
       total ram: C kbytes
       total time: D milliseconds
       duplicate: E pages
       normal: F pages
       normal bytes: G kbytes
       cache size: H bytes
       xbzrle transferred: I kbytes
       xbzrle pages: J pages
       xbzrle cache miss: K pages
       xbzrle cache miss rate: L
       xbzrle encoding rate: M
       xbzrle overflow: N

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 (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(".");
        }
    }