How do I get TotalStorageSpace or UsedStorageSpace metric from AWS RDS?

Question

I see that AWS RDS provides a FreeStorageSpace metric for monitoring disk usage. Now I am trying to create a generic pre-emptive alert for all my RDS but setting up an ideal threshold on FreeStorageSpace is not making sense.

For example, 20G might be a good threshold with RDS having total disk space as 100G but might be misleading for a RDS with total disk space of 40G.

So I was wondering if there is a way to get TotalStorageSpace or UsedStorageSpace metric from RDS (directly or indirectly).

Update

Since the fact is established that FreeStorageSpace is the only metric RDS provides related to disk storage, any ideas on if / how we can we build a custom metric for TotalStorageSpace or UsedStorageSpace?

p.s.: Creating separate alarms for each RDS for evaluating disk usage percentage seems such waste of time and resource.

Answer 1

If you enable Enhanced Monitoring, then the RDSOSMetrics log group in Cloudwatch Logs will have detailed JSON log messages which include filesystem statistics. I ended up creating a Cloudwatch Logs metric filter to parse out the usedPercent value from the fileSys attribute for the root filesystem. At least for Postgresql, these detailed logs include both / and /rdsdbdata filesystems; the latter is the one that is of interest in terms of storage space.

You can create a metric filter of the form {$.instanceID = "My_DB_Instance_Name" && $.fileSys[0].mountPoint = "/rdsdbdata"} and a corresponding metric value $.fileSys[0].usedPercent to get the used storage percentage for a given instance. This would then be available as a Log Metric that you could use to trigger an alarm. You probably need to create another metric replacing filesystem[0] with filesystem[1] since ordering is unknown for that array. You'd probably want to create these for each RDS instance you have so you know which one is running out of space, but you question seems to indicate you don't want a per-instance alarm.

I suppose you could exclude the $.instanceID from the metric filter and just get all values written to a single metric. When it reached a threshold and triggered an alarm, you'd need to start checking to see which instance is responsible.

Answer 2

According to the doc FreeStorageSpace is the only StorageSpace metrics you can get.

I can only assume that their logic is that you know what is your total space and having the FreeStorageSpace value you can also calculate how much is used.