A swap file on a database host is a slow outage

Swapping a database’s buffer pool to disk turns a memory pressure problem into a latency problem that is much harder to diagnose.

$ vmstat 1 5
procs -----------memory---------- ---swap-- -----io----
 r  b   swpd   free   buff  cache   si   so    bi    bo
 2  1 412008  88104  12044 188402  412  188  4102  2841

# si/so non-zero on a database host is the finding.

$ sysctl vm.swappiness
vm.swappiness = 60      # → 1, not 0

# 0 disables swap entirely and invites the OOM killer,
# which is a fast outage instead of a slow one.

Setting swappiness to 1 rather than 0 is the detail worth knowing: zero means the kernel will not swap even to avoid killing a process, which trades a gradual degradation for an abrupt one. Sizing the buffer pool so that swapping never becomes necessary is the actual fix, and the swappiness setting is a guard rail rather than a solution.