LVS

Revision as of 11:10, 16 July 2010

HOWTO

Pool or depool hosts

For pmtpa: edit the files in /etc/pybal/ directly. PyBal will reread the file within a minute.

For esams: edit the files in /home/w/conf/pybal/esams/ and wait a minute - PyBal will fetch the file over HTTP.

LVS installation

esams has a newer setup that uses Puppet and automatic BGP failover. Puppet arranges the service IP configuration, and installation of packages. To configure the service IPs that an LVS balancer should serve (both primary and backup!), set the $lvs_realserver_ips variable:

node /amslvs[1-4]\.esams\.wikimedia\.org/ {
        $cluster = "misc_esams"

        $lvs_balancer_ips = [ "91.198.174.2", "91.198.174.232", "91.198.174.233", "91.198.174.234" ]

        include base,
                ganglia,
                lvs::balancer

In this setup, all 4 hosts amslvs1-amslvs4 are configured to serve all three service IPs, although in practice every service IP is only ever serviced by one out of two hosts due to the router configuration.

Puppet uses the (now misleadingly named) wikimedia-lvs-realserver package to bind these IPs to the loopback (!) interface. This is to make sure that a server answers on these IPs, but does not announce them via ARP - we'll use BGP for that.

PyBal configuration

Puppet currently does not configure 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/cluster/ 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

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.