LVS

LVS installation

Service IP configuration

In the new BGP setup, service IPs should be installed as /32s bound to the loopback interface. The best way to do that is to install the - now somewhat misleadingly named - package wikimedia-lvs-realserver.

Enter the IP addresses in the dialog as prompted on installation, or edit /etc/default/wikimedia-lvs-realserver and run dpkg-reconfigure wikimedia-lvs-realserver.

Remove unnecessary networking modules

A default Ubuntu install has iptables and xtables modules loaded, which slow LVS operation down somewhat, and are unnecessary. To disable them with the next boot, put a file /etc/modprobe.d/iptables with the following content:

alias ip_tables off
alias iptable_filter off
alias x_tables off

Install PyBal

# apt-get install pybal

Edit the configuration in /etc/pybal/pybal.conf. This a .INI-format file, with one section for each LVS service, and a [global] section with global settings. The default configuration file should provide hints.

Put a local copy of the conf files in /home/wikipedia/conf/pybal under the hostname; see the README.txt in that directory.

There is more information on Pybal if you need it.

BGP failover and load sharing

Example BGP configuration:

 neighbor 10.0.0.210 remote-as 64601
 neighbor 10.0.0.210 description "PyBal on lvs3.wikimedia.org"
 neighbor 10.0.0.210 timers  keep-alive 10  hold-time 30
 neighbor 10.0.0.210 update-source loopback 1
 neighbor 10.0.0.210 prefix-list LVS in
 neighbor 10.0.0.210 prefix-list none out

Prefix-list LVS:

ip prefix-list LVS: 2 entries
     seq 5 permit 208.80.152.0/22 ge 32 
     seq 10 permit 10.0.0.0/8 ge 32 

SSH checking

As the Apache cluster is often suffering from broken disks which break SSH but keep Apache up, I have implemented a RunCommand monitor in PyBal which can periodically run an arbitrary command, and check the server's health by the return code. If the command does not return within a certain timeout, the server is marked down as well.

The RunCommand configuration is in /etc/pybal/pybal.conf:

runcommand.command = /bin/sh
runcommand.arguments = [ '/etc/pybal/runcommand/check-apache', server.host ]
runcommand.interval = 60
runcommand.timeout = 10

runcommand.command 

The path to the command which is being run. Since we are using a shell script and PyBal does not invoke a shell by itself, we have to do that explicitly.

runcommand.arguments 

A (Python) list of command arguments. This list can refer to the monitor's server object, as shown here.

runcommand.interval 

How often to run the check (seconds).

runcommand.timeout 

The command timeout; after this amount of seconds the entire process group of the command will be KILLed, and the server is marked down.

Currently we're using the following RunCommand script, in /etc/pybal/runcommand/check-apache:

#!/bin/sh

set -e

HOST=$1
SSH_USER=pybal-check
SSH_OPTIONS="-o PasswordAuthentication=no -o StrictHostKeyChecking=no -o ConnectTimeout=8"

# Open an SSH connection to the real-server. The command is overridden by the authorized_keys file.
ssh -i /root/.ssh/pybal-check $SSH_OPTIONS $SSH_USER@$HOST true

exit 0

The limited ssh accounts on the application servers are managed by the wikimedia-task-appserver package.

Old

To install an LVS load balancer, on a base Ubuntu install, do:

1. apt-get install pybal (ignore the warning about the kernel not supporting IPVS)
2. Set up configuration in /etc/pybal/
3. Restart PyBal and check whether it is working correctly (tail /var/log/pybal.log)
4. Bind the LVS ip(s) to the external interface (usually eth0); for persistence after booting add the following line to the loopback interface block in /etc/network/interfaces:

up ip addr add ip/32 dev $IFACE

Removing real-servers

Real-servers can be removed from the pool temporarily by simply shutting down apache. Because lvsmon runs in a single thread, checking apaches in turn, it's probably better to remove permanently dead apaches from the apache nodelist.

If a misbehaving realserver is in LVS and for some reason PyBal is not removing it, you can remove it by running a command of the following form:

ipvsadm -d -t <VIP>:<PORT> -r <REALSERVER>

e.g.

ipvsadm -d -t 66.230.200.228:80 -r sq1.pmtpa.wmnet

Diagnosing problems

Run ipvsadm -l on the director. Healthy output looks like this:

IP Virtual Server version 1.2.1 (size=4096)
Prot LocalAddress:Port Scheduler Flags
  -> RemoteAddress:Port           Forward Weight ActiveConn InActConn
TCP  upload.pmtpa.wikimedia.org:h wlc
  -> sq10.pmtpa.wmnet:http        Route   10     5202       5295
  -> sq1.pmtpa.wmnet:http         Route   10     8183       12213
  -> sq4.pmtpa.wmnet:http         Route   10     7824       13360
  -> sq5.pmtpa.wmnet:http         Route   10     7843       12936
  -> sq6.pmtpa.wmnet:http         Route   10     7930       12769
  -> sq8.pmtpa.wmnet:http         Route   10     7955       11010
  -> sq2.pmtpa.wmnet:http         Route   10     7987       13190
  -> sq7.pmtpa.wmnet:http         Route   10     8003       7953

All the servers are getting a decent amount of traffic, there's just normal variation.

If a realserver is refusing connections or doesn't have the VIP configured, it will look like this:

IP Virtual Server version 1.2.1 (size=4096)
Prot LocalAddress:Port Scheduler Flags
  -> RemoteAddress:Port           Forward Weight ActiveConn InActConn
TCP  upload.pmtpa.wikimedia.org:h wlc
  -> sq10.pmtpa.wmnet:http        Route   10     2          151577
  -> sq1.pmtpa.wmnet:http         Route   10     2497       1014
  -> sq4.pmtpa.wmnet:http         Route   10     2459       1047
  -> sq5.pmtpa.wmnet:http         Route   10     2389       1048
  -> sq6.pmtpa.wmnet:http         Route   10     2429       1123
  -> sq8.pmtpa.wmnet:http         Route   10     2416       1024
  -> sq2.pmtpa.wmnet:http         Route   10     2389       970
  -> sq7.pmtpa.wmnet:http         Route   10     2457       1008

Active connections for the problem server are depressed, inactive connections normal or above normal. This problem must be fixed immediately, because in wlc mode, LVS load balances based on the ActiveConn column, meaning that servers that are down get all the traffic.

LVS director list

Cluster	Director	VIP
pmtpa apaches	lvs3	10.2.1.1
search backend 1	lvs3	10.2.1.11
search backend 2	lvs3	10.2.1.12
search backend 3	lvs3	10.2.1.13
rendering	lvs3	10.2.1.21
pmtpa text	lvs4	208.80.152.2
pmtpa upload	lvs2	208.80.152.3
esams text	amslvs3	91.198.174.232
esams upload	amslvs2	91.198.174.234
esams bits	amslvs1	91.198.174.233

A good way to generate this list is:

 dsh -N ALL -f -e 'ipvsadm -l ' 

and look for the hosts that give you a pile of output. Because most hosts have config files for both text and upload squids, they will pretend to serve for both. You can check what they are really doing by looking at the output.

Example: output like

fuchsia:  	IP Virtual Server version 1.2.1 (size=1048576)
fuchsia:  	Prot LocalAddress:Port Scheduler Flags
fuchsia:  	  -> RemoteAddress:Port           Forward Weight ActiveConn InActConn
fuchsia:  	TCP  rr.esams.wikimedia.org:www wlc
fuchsia:  	  -> knsq6.esams.wikimedia.org:ww Route   10     26707      31425     
fuchsia:  	  -> knsq5.esams.wikimedia.org:ww Route   10     26708      31426     
fuchsia:  	  -> knsq24.esams.wikimedia.org:w Route   10     26741      31116     
... (more lines with lots of ActiveConn)
fuchsia:  	TCP  upload.esams.wikimedia.org:w wlc
fuchsia:  	  -> knsq17.esams.wikimedia.org:w Route   10     0          5         
fuchsia:  	  -> knsq13.esams.wikimedia.org:w Route   10     0          5         
fuchsia:  	  -> knsq19.esams.wikimedia.org:w Route   10     0          5         
... (more lines with 0 ActiveConn)

means that the host is doing lvs for rr.esams.wikimedia.org but not for upload.esams.wikimedia.org.