I/O Scheduler

The I/O Scheduler is the Linux kernel component that manages the queue of read and write requests to block devices (disks). It decides the execution order of requests to optimize throughput and minimize latency.

How it works #

Linux offers several schedulers: cfq (Completely Fair Queuing, for desktops), deadline/mq-deadline (for servers and databases), noop/none (for SSD/NVMe). For Oracle the recommendation is deadline, which serves requests minimizing disk seeks. It is configured via /sys/block/sdX/queue/scheduler and made permanent via GRUB.

What it’s for #

The default cfq distributes I/O equally among processes — ideal for a desktop, terrible for a database that needs priority on critical I/O requests. deadline ensures no request stays in queue too long, reducing db file sequential read latency.

What can go wrong #

Leaving the default (cfq or bfq on some systems) means Oracle competes for I/O with all other system processes. On a dedicated database server this is wasteful: the database should have absolute priority on disk operations.