The following is a detailed guide to get Redis installed and configured for your server. It assumes that you are an Ubuntu Server 14.04, or the equivalent Debian release.
First, download the Drupal redis module, which should go to sites/all/modules/contrib. You can do that in many ways, here is how you would use Drush for that:
drush @live dl redis
You do not need to enable any Redis modules in Drupal.
Then, install the Redis Server itself. On Debian/Ubuntu you can do the following. On CentOS/RedHat, you should use yum.
aptitude install redis-server
Then, install PHP's Redis integration. Once you do that, you do not need to compile from source, or anything like that, as mentioned in Redis README.txt file.
aptitude install php5-redis
Restart PHP, so it loads the Redis integration layer.
This assumes you are using PHP FPM:
service php5-fpm restart
If you are using PHP as an Apache module, then you need to restart it as follows:
service apache2 restart
Then in your settings.php file, you should replace the section for memcache which would be as follows:
$conf['cache_backends'] = './sites/all/modules/contrib/memcache/memcache.inc';
$conf['cache_default_class'] = 'MemCacheDrupal';
$conf['memcache_servers'] = array('127.0.0.1:11211' => 'default');
$conf['memcache_key_prefix'] = 'site1';
And replace it with the following configuration lines:
// Redis settings
$conf['redis_client_interface'] = 'PhpRedis';
$conf['redis_client_host'] = '127.0.0.1';
$conf['lock_inc'] = 'sites/all/modules/contrib/redis/redis.lock.inc';
$conf['path_inc'] = 'sites/all/modules/contrib/redis/redis.path.inc';
$conf['cache_backends'] = 'sites/all/modules/contrib/redis/redis.autoload.inc';
$conf['cache_default_class'] = 'Redis_Cache';
// For multisite, you must use a unique prefix for each site
$conf['cache_prefix'] = 'site1';
Changing Redis Configuration
You can then review the /etc/redis/redis.conf file to see if you should tweak parameters more, such as changing maxmemory to limit it to a certain amount, as follows:
More below on this specific value.
Checking That Redis Is Working
To check that Redis is working, you can inspect that keys are being cached. For this, you can use the redis-cli tool. This tool can be used interactively, as in, you get a prompt and type commands in it, and results are returned. Or you can use the specific command as an argument to redis-cli.
For example, this command filters on a specific cache bin, the cache_bootstrap one:
127.0.0.1:6379> keys *cache_boot*
Or you can type it as:
$ redis-cli keys "*cache_boot*"
In either case, if Drupal is caching correctly, you should see output like this:
As you can see, the key structure is simple, it is composed of the following components, separated by a colon:
This is the site name in a multi site environment.
This is the cache table name when using the default database caching in Drupal.
This is the unique name for the cached item. For cached pages, the URL is used, with the protocol (http or https) and the host/domain name.
You can also filter by site, using the cache_prefix:
$ redis-cli keys "*site1:cache_page*"
The output will be something like this:
You can also check how many items are cached in the database:
$ redis-cli dbsize
The output will be the number of items:
Flushing The Cache
If you need to clear the cache, you can do:
$ redis-cli flushall
Checking Time To Live (TTL) For A Key
You can also check how long does a specific item stay in cache, in seconds remaining:
$ redis-cli ttl site1:cache_page:http://example.com/
The output will be the number of seconds:
Getting Redis Info
You can get a lot of statistics and other information about how Redis is doing, by using the info command:
$ redis-cli info
You can check the full documentation for the info command.
But here is one of the important values to keep an eye on is used_memory_peak_human, which tells you the maximum memory that was used given your site's specifics, such as the number of items cached, the rate of caching, the size of each item, ...etc.
You can use that value to tune the maxmemory parameter, as above.
You can decrease the Minimum Cache Lifetime under /admin/config/development/performance to make the available memory fit that number, or the other way around: you can allocate more memory to fit more.
Monitoring Redis Operations In Real Time
And finally, here is a command that would show you all the operations that are being done on Redis in real time. Do not try this on a high traffic site!
$ redis-cli monitor
Redis performance as a page cache for Drupal is quite good, with Time To First Byte (TTFB) is ~ 95 to 105 milliseconds.