MySQL Relational Database Service on AWS

Amazon Web Services Logo The stable of services available through AWS is continuing to expand! Last night Amazon announced RDS (Relational Database Service) which look a lot like EC2 instances running MySQL with EBS volumes – something I have a fair bit of experience with. However, these have the added benefit of being a service that can scale memory and processor both up and down with a single service call.

# ds-modify-db-instance mydbinstance --db-instance-class db.m1.xlarge -s 100

This flexibility comes with a downside, namely a 4 hour monthly service window where patches, updates and those requested capacity changes are applied. You can choose to apply them immediately, but your application should be prepared to handle the downtime. What happens is, your database instance goes offline and when it comes back, it has all the changes you requested applied. So at best, you should expect uptime in the 99.4% range. Most applications can handle a 4 hour downtime if it’s planned for. Under more conventional MySQL builds, developers or system administrators will mitigate these downtimes by first applying changes to slaves, promotion of one slave to master and then finally applying the changes to the original master. This sort of safety net provides gives applications smaller downtime windows (at most a few minutes each) allowing for theoretical 99.999% uptime.

Transitioning to RDS may not be without pain either. Importing your data is done through a mysqldump (or other flatflile export) and then playing that file back into your AWS instance. Depending on the size of your dataset a full mysqldump and re-importing may take days (no I’m not exaggerating). Also note, during the time mysqldump runs, your original database will acquire a read lock for consistency. With some DB’s I manage, I’ve stopped using MySQL dump entirely because the dumps took more than 4 hours to complete on a dedicated slave. With the myriad of snapshotting technologies available, it’s much easier to grab a binary copy of the DB every few hours. One last limitation is replication isn’t an option. I suspect AWS will be working on this soon as part of a HA (High Availability) release option.

Despite the limitations, I’m excited about this offering. This offloads much of the maintenance and management tasks which are usually the most tedious. I also hope that this means a higher IO disk subsystem may be coming to EBS soon.

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