Even with SSD becoming more prolific for local-ish storage even on cloud servers, we still encounter attached storage (SAN) quite frequently. This can be fine, but we often find that the performance of SANs is quite dismal.
- SANs are very efficient with bulk sequential reads or writes, which a database server of course doesn’t care about;
- SANs often have a large memory cache (sometimes with intermediate SSD), optimising frequent reads – again a database server won’t benefit from this, because it will already have any recent data in its own caches (for MySQL/MariaDB, that’d typically be the InnoDB Buffer Pool). So usually any database disk read needs to access physical storage = slow;
- SANs will cache data writes in (battery backed) memory. That can work fine, provided they respond quickly enough.
Slow response on that last aspect can really damage performance. But we need …[Read more]