a9s Parachute Concerns

Every a9s Data Services Service Instance has an a9s Parachute process running. This process will stop the node process when the persistent disk hits the configured usage limit (default: 80%). This is valid for all Services.

The persistent disk is mounted using the ext4 file system, that reserves a portion of the file system (5% by default) for the root user to prevent, among other things, normal users (e.g. vcap) from filling the disk and crashing the system. That means that the parachute usage limit should not be set to more than 95%, and perhaps anything above 90% is a bit unsafe. That’s because any non-root processes (e.g. postgres, logstash..) would stall before a9s Parachute triggers.

The calculation for the used disk space does not consider the reserved disk space, and it is calculated over the overall disk usage.