Migrating memcached

By: Ian Winter

Tags:

  • memcached
  • migration
  • tcpcopy

As part of the pre-Christmas push we rolled out a series of infrastructure improvements including both new additional servers and new servers to replace old ones. One set of servers to be upgraded were our cache servers. These have two roles day-to-day, firstly they’re our Akamai (CDN) origin servers and secondly they store our primary memcached instances.

Each server has multiple instances of memcached running on different ports. There are multiple sets for both the “data” and “view” segments. Each of the new servers has 64GB RAM.

Migration Challenges

The main challenges when planning to migrate the data is mitigating the impact on the platform. memcached is a core element. It’s used to store HTML fragment caches and various parts of both site and member data.

There is a tight line between getting the migration completed versus keeping the application as quick as possible during the whole procedure.

Approaches

There are a few approaches which are briefly outlined below:

  • Big Bang. Update all the configuration files and simply push that code live. This has a huge hit on performance as all the new instances are cold, it is however the quickest option.
  • Migration instance by instance. Each instance would be moved and relevant configuration files pushed live. This way only 1 or X instances is “cold”. This limits impact, but, also takes time as only 1 instance can be done at any one time.
  • Write to both. The application code could be altered to make all writes to both the existing set of instances and the new. This isn’t great as we’d need a full QA cycle to validate the code works. It’s also something we’d have to implement then pull back out down the line.
  • Mirror traffic. Similar to the above, but, this time lower down the stack. Essentially duplicate all the TCP level traffic so it warms in parallel and keeps both instances in sync meaning new writes, deletes occur on both existing and new sets.

Solution

In the end the best solution which minimised the impact to the platform was to mirror all the traffic to the existing instances across to the new instances. tcpcopy helped us to get this done. As well as migrating data, tcpcopy could be used to load test, stress test and benchmark.

The plan involved running the tcpcopy for 24 hours, see how full the new instances reached then decided if another 24 hours should elapse before the code changes to point at new instances are deployed.

There would be some data “missing” however all freqently used data keys, core and shared information will all exist and as such mitigate the performance concerns.

tcpcopy

Installing tcpcopy is pretty straight forward. Assuming all requirements are in place:

wget --no-check-certificate \
  https://github.com/downloads/wangbin579/tcpcopy/tcpcopy-0.6.5.tar.gz && \
  tar xvfz tcpcopy-0.6.5.tar.gz && cd tcpcopy-0.6.5 && \
  ./configure && make && make install

Once that’s installed on your target server (new cache servers) you’ll need to setup the iptables rules. Note there are a couple of extra’s here to ensure that connections from the localhost and from the office IP were not queued to allowing monitoring.

modprobe ip_queue;
iptables -A OUTPUT -o lo -j ACCEPT;
iptables -A OUTPUT -d 1.2.3.4/32 -j ACCEPT;
iptables -A OUTPUT -p tcp --match multiport --sports 11211:11212 -j QUEUE;

You can then fire up the intercepter, the -d option backgrounds the process as a daemon.

intercept -d

Next you’ll need to head over to the source servers. In our case this is the existing cache servers.

tcpcopy -d -x 11211-192.168.2.184:11211
tcpcopy -d -x 11212-192.168.2.184:11212

The format of the copy is localport-targetip:targetport. You can tweak with more options as well, see the manual for full details. You can also include logging if needed by using -l /path/to/log.

Monitoring

In order to see how the copies were going 3 monitoring methods were used.

Direct monitoring via telnet

[root@server ~] telnet 0 11211
Trying 0.0.0.0...
Connected to 0.
Escape character is '^]'.
stats
STAT pid 1234
STAT uptime 1234
STAT time 111111111
...

Ruby key dumper

This small Ruby key dumper allows you to dump out a selection of the keys. It’s not great and memcached doesn’t allow a full dump of keys in anyway. It allows you to see what’s there and validate it’s content though.

Overall instance stats from memcache-top

This is a perl program available on google code which allows you monitor a given set of instances, much like regular top.


About the Author

Ian Winter

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