RDSDiskSpaceLimit #

Meaning #

Alert is triggered when RDS instance is low on storage.

Impact #

The PostgreSQL instance will stop to prevent data corruption if no more disk space is available.

Diagnosis #

Determine whether it’s a long-term growth trend requiring storage increase or abnormal disk usage reflecting another problem:

1. Check database size growth over the last 24 hours to identify abnormal disk usage growth

   Look at Storage usage panel in RDS instance details Grafana dashboard

2. Check if there are long-running transactions (or queries)

   Look at Postgresql live activity Grafana dashboard to find long-running query

   Why?

   A long running transaction (or query) forces PostgreSQL to maintain visibility of all rows since the beginning of the transaction. So PostgreSQL accumulate WAL files until the transaction is close.

   PostgreSQL

   SELECT
       pid,
       leader_pid,
       datname as database,
       usename AS user,
       application_name,
       EXTRACT(EPOCH FROM now() - xact_start) as transaction_duration,
       CASE
           WHEN state = 'active' THEN EXTRACT(EPOCH FROM now() - query_start)
           ELSE null
       END query_duration,
       state,
       wait_event_type,
       wait_event,
       CASE
           WHEN state = 'active' THEN query
           ELSE null
       END query,
       xact_start as transaction_start,
       CASE
           WHEN state = 'active' THEN query_start
           ELSE null
       END query_start,
       CASE
           WHEN state != 'active' THEN EXTRACT(EPOCH FROM state_change - query_start)
           ELSE null
       END last_query_duration,
       CASE
           WHEN state != 'active' THEN query
           ELSE null
       END last_query,
       pg_blocking_pids(pid) as blocked_by,
       client_addr,
       backend_start,
       ceil(EXTRACT(EPOCH FROM now() - backend_start)) as backend_duration
   FROM pg_stat_activity
   WHERE query_start is not null
       AND usename!='rdsrepladmin'
       AND query not like 'START_REPLICATION %'
       AND pid != pg_backend_pid();
   

3. Check the status of PostgreSQL replication slots

   Why?

   PostgreSQL keeps WAL files on its disk until the replication slot client acknowledges they consumed it.

4. Check log file sizes

   Why?

   PostgreSQL logs could consume large disk space.

   Usually related to:

   • Connection/disconnections
   • Slow queries
   • PGaudit logs
   • Internal errors
   • Temporary files

   PostgreSQL parameters:

   • log_temp_files can be set to log temporary file creation above a size threshold
   • temp_file_limit parameter can be set to avoid over-usage: https://www.postgresql.org/docs/16/runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-DISK

Mitigation #

You must avoid reaching no disk space left situation.

• Fix the system that blocks PostgreSQL to recycle its WAL files

  • If long-running transactions/queries: Cancel or kill the transactions
  • If non-running replication slot: Delete replication slot

• Increase RDS disk space

  AWS RDS limitations

  • Minimal disk increase is 10%

  • You can’t make further storage modifications for either six (6) hours or until storage optimization has completed on the instance, whichever is longer.

  • Storage optimization can take several hours

  • Storage can’t be reclaimed

  See more on AWS documentation

Additional resources #

n/a