Document Redis session cleanup

doc/operations/README.md

 # GitLab operations
 
 - [Sidekiq MemoryKiller](sidekiq_memory_killer.md)
+- [Cleaning up Redis sessions](cleaning_up_redis_sessions.md)

doc/operations/cleaning_up_redis_sessions.md

+# Cleaning up stale Redis sessions
+
+Since version 6.2, GitLab stores web user sessions as key-value pairs in Redis.
+Prior to GitLab 7.3, user sessions did not automatically expire from Redis. If
+you have been running a large GitLab server (thousands of users) since before
+GitLab 7.3 we recommend cleaning up stale sessions to compact the Redis
+database after you upgrade to GitLab 7.3. You can also perform a cleanup while
+still running GitLab 7.2 or older, but in that case new stale sessions will
+start building up again after you clean up.
+
+In GitLab versions prior to 7.3.0, the session keys in Redis are 16-byte
+hexadecimal values such as '976aa289e2189b17d7ef525a6702ace9'. Starting with
+GitLab 7.3.0, the keys are
+prefixed with 'session:gitlab:', so they would look like
+'session:gitlab:976aa289e2189b17d7ef525a6702ace9'. Below we describe how to
+remove the keys in the old format.
+
+First we define a shell function with the proper Redis connection details.
+
+```
+rcli() {
+  # This example works for Omnibus installations of GitLab 7.3 or newer. For an
+  # installation from source you will have to change the socket path and the
+  # path to redis-cli.
+  sudo /opt/gitlab/embedded/bin/redis-cli -s /var/opt/gitlab/redis/redis.socket "$@"
+}
+
+# test the new shell function; the response should be PONG
+rcli ping
+```
+
+Now we do a search to see if there are any session keys in the old format for
+us to clean up.
+
+```
+# returns the number of old-format session keys in Redis
+rcli keys '*' | grep '^[a-f0-9]\{32\}$' | wc -l
+```
+
+If the number is larger than zero, you can proceed to expire the keys from
+Redis. If the number is zero there is nothing to clean up.
+
+```
+# Tell Redis to expire each matched key after 600 seconds.
+rcli keys '*' | grep '^[a-f0-9]\{32\}$' | awk '{ print "expire", $0, 600 }' | rcli
+# This will print '(integer) 1' for each key that gets expired.
+```
+
+Over the next 15 minutes (10 minutes expiry time plus 5 minutes Redis
+background save interval) your Redis database will be compacted. If you are
+still using GitLab 7.2, users who are not clicking around in GitLab during the
+10 minute expiry window will be signed out of GitLab.