Redis (1.2.6) : Slow queries

Question

Hi All,

We are using Redis 1.2.6 in production environment. There are 161804 keys in redis. Machine has 2GB RAM.

Problem:

Select queries to Redis are taking 0.02 sec on an average. But some times they take 1.5-2.0 secs, I think whenever redis save modified keys on disk.

One strange thing I noticed before and after restarting the redis is that:

Before restart "changes_since_last_save" changing too fast and was reaching 3000+ (in 5 minutes). But after restart "changes_since_last_save" remains below 20 or so.

Redis stats before restart:

{:bgrewriteaof_in_progress=>"0", :arch_bits=>"64", :used_memory=>"53288487", :total_connections_received=>"586171", :multiplexing_api=>"epoll", :used_memory_human=>"50.82M", :total_commands_processed=>"54714152", :uptime_in_seconds=>"1629606", :changes_since_last_save=>"3142", :role=>"master", :uptime_in_days=>"18", :bgsave_in_progress=>"0", :db0=>"keys=161863,expires=10614", :connected_clients=>"13", :last_save_time=>"1280912841", :redis_version=>"1.2.6", :connected_slaves=>"1"}

Redis stats after restart:

{:used_memory_human=>"49.92M", :total_commands_processed=>"6012", :uptime_in_seconds=>"1872", :changes_since_last_save=>"2", :role=>"master", :uptime_in_days=>"0", :bgsave_in_progress=>"0", :db0=>"keys=161823,expires=10464", :connected_clients=>"13", :last_save_time=>"1280917477", :redis_version=>"1.2.6", :connected_slaves=>"1", :bgrewriteaof_in_progress=>"0", :arch_bits=>"64", :used_memory=>"52341658", :total_connections_received=>"252", :multiplexing_api=>"epoll"}

Not sure what is going wrong here.

Thanks in advance.

Sunil

Answer 1

By default Redis is configured to dump all data to disk from time to time depending on the amount of keys that changed in a time span (see the default config).

Another option is to use the append-only file, which is more lightweight but needs some kind of maintenance – you need to run BGREWRITEAOF every once in a while so that your log doesn't get too big. There's more on the Redis config file about this.

As Tobias says, you should switch to 2.0 as soon as you can since it's faster and, in many cases, uses less memory than 1.2.6.