local-storage.txt

== Local Storage

This chapter outlines specific performance considerations for local
storage in high-availability configurations.

=== RAID Level

TODO

=== RAID Stripe Size

TODO

=== RAID Scrubbing/Resilvering

Some RAID configurations periodically check the consistency of
individual arrays by doing a sector-by-sector comparison of the
mirrors. This process is variously known as consistency checking,
scrubbing or resilvering. It is not only time-consuming, but can also
cause a massive performance drop on the array being checked --
throughput being reduced by as much as 90% is not uncommon.

For software RAID on Linux, this recurring behavior is governed by a
+cron+ job that invokes the +raid-check+ utility, and which typically
runs once a week by default. Hardware based RAID solutions may feature
similar checks, typically configurable via the RAID BIOS or a storage
administration utility from the vendor.

Consider reducing the frequency of such consistency checks, or at
least schedule them during a time when they are expected to have
little impact on your application (such as periods of lower
utilization).