COLO - Coarse Grain Lock Stepping

COLO or Coarse Grain Lock Stepping is an High Availability solution that builds on top of Remus. Remus was prepared for use with COLO in Xen 4.5. The COLO Manager component is part of Xen 4.7, while other components will eventually be part of QEMU. 
= Background =
COLO FT/HA (COarse-grain LOck-stepping Virtual Machines for Non-stop Service) project is a high availability solution. Both primary VM (PVM) and secondary VM (SVM) run in parallel. They receive the same request from client, and generate response in parallel too. If the response packets from PVM and SVM are identical, they are released immediately. Otherwise, a VM checkpoint (on demand) is conducted. The idea is presented in Xen summit 2012, and 2013, and academia paper in SOCC 2013.
= Components =
* COLO Manager:
: COLO Checkpoint/Failover Controller 
: Modifications of save/restore flow to realize continuous migration, to make sure the state of VM in Secondary side 
: always be consistent with VM in Primary side.
* COLO Block Replication: (Please refer to BlockReplication)
: When primary VM writes data into image, the colo disk manger captures this data
: and send it to secondary VM’s which makes sure the context of secondary VM's image is consentient with
: the ontext of primary VM 's image.
* COLO Proxy:
: We need an module to compare the packets returned by Primary VM and Secondary VM
: and decide whether to start a checkpoint according to some rules. 
: In previous version. COLO Proxy is a linux kernel module for host. Because of this module 
: can't accepted by kernel community,you have to apply some patches to linux kernel and compile it yourself. 
: No support has been provided to kernelspace COLO Proxy.
: In current version.COLO Proxy is a part of Qemu, all codes running in userspace. The filter-redirector, filter-mirror, 
: colo-compare and filter-rewriter compose the COLO-proxy. We can get better performance with COLO-proxy's help.
: Compare with previous kernelspace COLO-Proxy, no more additional job to do, you just need ensure your qemu version support COLO Proxy. 
= Current Status =
COLO (based on xm) has already been in development for over three years. A paper has come out at 2013. Since XEN has deprecated xm and turn to xl, we are implementing COLO on xl now.
The overall status of COLO:
* COLO Manager (Merged since 4.7.0-rc1):
* COLO Block replication (Merged by Qemu)
* COLO Proxy (Userspace COLO Proxy has been merged by Qemu)
: The legacy kernelspace COLO Proxy View on Github 
= Requirements =
== Hardware requriements ==
There is at least one directly connected nic to forward the network requests from client
to secondary vm. The directly connected nic must not be used by any other
purpose. If your guest has more than one nic, you should have directly
connected nic for each guest nic. If you don't have enough directly connected
nic, you can use vlan.
== Dom0 requirements ==
# Kernel with dom0 support
# If your host os has OEM-released xen tools, please uninstall it
# Use latest Qemu version call for Xen
:: (Because have some COLO module communication related patches haven't be merged, you can use internal version: View on Github )
== Guest requirements ==
Only HVM guest(without pv extensions) is supported now. If you want to use OEM released guest os, please use SUSE. REDHAT and Ubuntu is not supported now because I don't find any way to disable pv extensions. If you want to use REDHAT or Ubuntu, you need to build the newest kernel which has the parameter xen_nopv.
= Setup COLO environment =
== Network link topology ==
=================================normal ======================================
 +--------+
 |client |
 master +----+---+ slave
-------------------------+ | + -------------------------+
 PVM | + | |
+-------+ +----[eth0]-----[switch]-----[eth0]---------+ |
|guest | +---+-+ | | +---+-+ |
| [tap0]--+ br0 | | | | br0 | |
| | +-----+ [eth1]-----[forward]----[eth1]--+ +-----+ SVM |
+-------+ | | | +-------+|
 | | | +-----+ | guest ||
 [eth2]---[checkpoint]---[eth2] +--+br1 |-[tap0] ||
 | | +-----+ | ||
 | | +-------+|
-------------------------+ +--------------------------+
e.g.
master:
br0: 192.168.2.98
eth1: 192.168.1.33
eth2: 192.168.3.1
slave:
br0: 192.168.2.99
br1: no ip address
eth1: 192.168.1.88
eth2: 192.168.3.2
===========================after failover=====================================
 +--------+
 |client |
 master (dead) +----+---+ slave (alive)
-------------------------+ | ---------------------------+
 PVM | + | |
+-------+ +----[eth0]-----[switch]-----[eth0]-------+ |
|guest | +---+-+ | | +---+-+ |
| [tap0]--+ br0 | | | | br0 +--+ |
| | +-----+ [eth1]-----[forward]----[eth1] +-----+ | SVM |
+-------+ | | | +-------+|
 | | +-----+ | | guest ||
 [eth2]---[checkpoint]---[eth2] |br1 | +[tap0] ||
 | | +-----+ | ||
 | | +-------+|
-------------------------+ +--------------------------+
== Test environment prepare ==
On both Primary/Secondary hosts:
* checkout necessary repos:
# cd ~
# git clone https://github.com/xen-project/xen.git
# git clone https://github.com/zhangckid/qemu.git
* Build and install xen
# cd ~/xen 
# ./autogen.sh; ./configure --enable-debug
# make dist-xen; make install-xen
# make dist-tools; make install-tools
* Build qemu
# cd ~/qemu
# git checkout qemu-colo-for-xen
# cd ~/xen/tools/qemu-xen-dir
# ./configure --enable-xen --target-list=x86_64-softmmu \
 --extra-cflags="-I~/xen/tools/include -I~/xen/tools/libxc -I~/xen/tools/xenstore" \
 --extra-ldflags="-L~/xen/tools/libxc -L~/xen/tools/xenstore"
# make $(grep -c processor /proc/cpuinfo)
Note: You must use qemu that we provide, qemu-xen and qemu-xen-traditional are not supported.
On Primary host:
* guest config
:Add "xen_platform_pci=0" and below disk/net config into the guest configfile.
disk = [ 'format=raw,devtype=disk,access=w,backendtype=qdisk,vdev=hda,colo,colo-host=192.168.3.2,colo-port=9001,colo-export=qdisk1,active-disk=/mnt/ramfs/active_disk.img,hidden-disk=/mnt/ramfs/hidden_disk.img,target=/home/zhangchen/suse-64hvm-zc.img']
vif = [ 'mac=00:16:4f:00:00:11, bridge=br0, model=rtl8139, colo_sock_sec_redirector0_id=red0,colo_sock_sec_redirector0_ip=192.168.2.98,colo_sock_sec_redirector0_port=9003,
colo_sock_sec_redirector1_id=red1,colo_sock_sec_redirector1_ip=192.168.2.98,colo_sock_sec_redirector1_port=9004,
colo_filter_sec_redirector0_queue=tx,colo_filter_sec_redirector0_indev=red0,colo_filter_sec_redirector1_queue=rx,
colo_filter_sec_redirector1_outdev=red1,colo_filter_sec_rewriter0_queue=all,colo_sock_mirror_id=mirror0,colo_sock_mirror_ip=192.168.2.98,
colo_sock_mirror_port=9003,colo_sock_compare_pri_in_id=compare0,colo_sock_compare_pri_in_ip=192.168.2.98,colo_sock_compare_pri_in_port=9021,
colo_sock_compare_sec_in_id=compare1,colo_sock_compare_sec_in_ip=192.168.2.98,colo_sock_compare_sec_in_port=9004,colo_sock_compare_notify_id=not1,
colo_sock_compare_notify_ip=192.168.2.98,colo_sock_compare_notify_port=9998,colo_sock_redirector0_id=compare_out,
colo_sock_redirector0_ip=192.168.2.98,colo_sock_redirector0_port=9005,colo_sock_redirector1_id=compare0-0,colo_sock_redirector1_ip=192.168.2.98,
colo_sock_redirector1_port=9021,colo_sock_redirector2_id=compare_out0,colo_sock_redirector2_ip=192.168.2.98,colo_sock_redirector2_port=9005,
colo_filter_mirror_queue=tx,colo_filter_mirror_outdev=mirror0,colo_filter_redirector0_queue=rx,colo_filter_redirector0_indev=compare_out,
colo_filter_redirector1_queue=rx,colo_filter_redirector1_outdev=compare0,colo_compare_pri_in=compare0-0,colo_compare_sec_in=compare1,
colo_compare_out=compare_out0,colo_compare_notify_dev=not1,colo_checkpoint_host=192.168.2.98,colo_checkpoint_port=9998' ]
* Copy physical machine disk image from Primary to Secondary，and make sure their absolute path are the same
Note: colo-port is the secondary host's IP, colo-port is the secondary host's NBD server port, forwarddev is the directly connected nic.
= Run COLO =
On both Primary/Secondary hosts:
# service xencommons start
On Secondary host:
excute the following script:
#! /bin/bash
active_disk=/mnt/ramfs/active_disk.img
hidden_disk=/mnt/ramfs/hidden_disk.img
local_img=~/suse-64hvm.img
tmp_disk_size=`./qemu-colo/qemu-img info $local_img |grep 'virtual size' |awk '{print $3}'
function create_image()
{
 ~/qemu/qemu-img create -f qcow2 $1 $tmp_disk_size 
}
function prepare_temp_images()
{
 grep -q "^none /mnt/ramfs ramfs" /proc/mounts
 if [[ $? -ne 0 ]]; then
 mount -t ramfs none /mnt/ramfs/ -o size=2G
 fi
 if [[ ! -e $active_disk ]]; then
 create_image $active_disk 
 fi
 if [[ ! -e $hidden_disk ]]; then
 create_image $hidden_disk
 fi
}
prepare_temp_images
Note: It is recommended to put active disk and hidden disk in ramdisk.
On Primary host:
# xl create -p <domconfig>
# xl pause <domconfig>
# xl remus -c -p -u <domconfig> 192.168.3.2
= Known problems =
* Secondary vm may crash due to triple fault.
Note: this problem doesn't happen every time. So you can run colo again to avoid this problem.
= Trouble shooting =
* If there's some error happend when staritng COLO, you can do:
# Make sure you have all necessary modules that DOM0 needed on both side.
# Make sure you have followed all the instructions in this README.
# Try to reboot both primary and secondary host.
# If you still have problems, collect the error logs and contact Zhang Chen(zhangckid@gmail.com), Xie Changlong(xiecl.fnst@cn.fujitsu.com),
# Wen Congyang(wency@cn.fujitsu.com), Yang Hongyang(imhy.yang@gmail.com) for help.
= Example =
If you use SLES11.3, you can get the detailed steps from the wiki: Setup COLO on SLES11 SP3
An example guest config:
builder='hvm'
memory='2048'
vcpus=2
cpus=['2','3']
name='hvm_nopv_colo'
device_model_version='qemu-xen'
device_model_override='/home/zhangckid/qemu/x86_64-softmmu/qemu-system-x86_64'
disk = [ 'format=raw,devtype=disk,access=w,backendtype=qdisk,vdev=hda,colo,colo-host=192.168.3.2,colo-port=9001,colo-export=qdisk1,active-disk=/mnt/ramfs/active_disk.img,hidden-disk=/mnt/ramfs/hidden_disk.img,target=/home/zhangchen/suse-64hvm-zc.img']
vif = [ 'mac=00:16:4f:00:00:11, bridge=br0, model=rtl8139, colo_sock_sec_redirector0_id=red0,colo_sock_sec_redirector0_ip=192.168.2.98,colo_sock_sec_redirector0_port=9003,
colo_sock_sec_redirector1_id=red1,colo_sock_sec_redirector1_ip=192.168.2.98,colo_sock_sec_redirector1_port=9004,
colo_filter_sec_redirector0_queue=tx,colo_filter_sec_redirector0_indev=red0,colo_filter_sec_redirector1_queue=rx,
colo_filter_sec_redirector1_outdev=red1,colo_filter_sec_rewriter0_queue=all,colo_sock_mirror_id=mirror0,colo_sock_mirror_ip=192.168.2.98,
colo_sock_mirror_port=9003,colo_sock_compare_pri_in_id=compare0,colo_sock_compare_pri_in_ip=192.168.2.98,colo_sock_compare_pri_in_port=9021,
colo_sock_compare_sec_in_id=compare1,colo_sock_compare_sec_in_ip=192.168.2.98,colo_sock_compare_sec_in_port=9004,colo_sock_compare_notify_id=not1,
colo_sock_compare_notify_ip=192.168.2.98,colo_sock_compare_notify_port=9998,colo_sock_redirector0_id=compare_out,
colo_sock_redirector0_ip=192.168.2.98,colo_sock_redirector0_port=9005,colo_sock_redirector1_id=compare0-0,colo_sock_redirector1_ip=192.168.2.98,
colo_sock_redirector1_port=9021,colo_sock_redirector2_id=compare_out0,colo_sock_redirector2_ip=192.168.2.98,colo_sock_redirector2_port=9005,
colo_filter_mirror_queue=tx,colo_filter_mirror_outdev=mirror0,colo_filter_redirector0_queue=rx,colo_filter_redirector0_indev=compare_out,
colo_filter_redirector1_queue=rx,colo_filter_redirector1_outdev=compare0,colo_compare_pri_in=compare0-0,colo_compare_sec_in=compare1,
colo_compare_out=compare_out0,colo_compare_notify_dev=not1,colo_checkpoint_host=192.168.2.98,colo_checkpoint_port=9998' ]
#-----------------------------------------------------------------------------
# boot on floppy (a), hard disk (c), Network (n) or CD-ROM (d) 
# default: hard disk, cd-rom, floppy
boot='c'
sdl=0
vnc=1
vnclisten=''
stdvga = 0 
serial='pty'
apic=1
apci=1
pae=1
extid=0
keymap='en-us'
localtime=1
hpet=1
usbdevice='tablet'
xen_platform_pci = 0 
= Man Pages =
* xl(1) search for Remus
* xl.conf(5) search for colo.default.proxyscript
= Links =
For more information see:
* Status Update of COLO Project (Nov 13, Video)
* Status Update of COLO Project (Nov 13, Presentation)
* COLO Intro (Long), 2012
* COLO Intro (Short), 2012
* COLO on QEMU/KVM
Category:High Availability
Category:Glossary
Category:Xen 4.5
Category:Xen 4.7