Recently, I faced a problem which memory usage on ElastiCache for Redis becomes huge.
This post describes how we can investigate such a problem (which keys are bottlenecks on ElastiCache).

Export and download a Redis backup

At first, to analyze, we have to download the .rdb file from our ElastiCache following the official guide.

When exporting an ElastiCache backup, please be careful that the region for S3 must be the same as the one for the backup.

Analyze .rdb with redis-rdb-tools

After downloading the file, we can analyze it with redis-rdb-tools.

For example, if we wanna generate a memory report, we can do it as follows.

> rdb -c memory /var/redis/6379/dump.rdb --bytes 128 -f memory.csv
> cat memory.csv

database,type,key,size_in_bytes,encoding,num_elements,len_largest_element
0,list,lizards,241,quicklist,5,19

If your .rdb is too large (it may be difficult to open the csv file), you could also generate a sampled report like:

> rdb -c memory /var/redis/6379/dump.rdb | ruby -ne 'print $_ if rand < 0.1' > memory.csv

Of course, redis-rdb-tools supports features other than generating a memory report.
For more information, please see the README.

Summary

• We can export and download a backup file easily with ElastiCache and S3.
• We can analyze a .rdb file easily with a tool like redis-rdb-tools.

References

• https://docs.aws.amazon.com/AmazonElastiCache/latest/red-ug/backups-exporting.html
• https://github.com/sripathikrishnan/redis-rdb-tools