Every block I/O request in Linux passes through an I/O scheduler before reaching the device. The scheduler's job is to reorder and merge requests to minimize seek time on spinning disks. For a rotational drive, sorting requests by sector (the classic elevator algorithm) can turn a 10ms seek storm into a smooth sweep across the platter. But NVMe drives don't seek — and the scheduler that helped HDDs actively hurts them.

Linux exposes the scheduler per-device at /sys/block/nvme0n1/queue/scheduler. The choices you'll see include none, mq-deadline, kyber, and bfq. Since kernel 5.0, everything runs on blk-mq (multi-queue block layer), where each CPU has its own submission queue and the device has multiple hardware queues that can be serviced in parallel.

mq-deadline maintains two sorted red-black trees (read and write) plus two FIFO lists with expiration deadlines (500ms reads, 5s writes). It serves sorted requests until a deadline expires, then switches to the FIFO head to prevent starvation. This is great for a SATA SSD or HDD where the device can process one request at a time and merging adjacent sectors saves real work.

none does nothing — it hands requests directly from the per-CPU submit queue to the device's hardware queue. On an NVMe drive with 64K hardware queue depth and 8+ parallel queues, this is exactly what you want. The device's internal FTL reorders better than the kernel can, and every cycle spent in the scheduler is pure overhead.

Real example: On a Samsung 980 Pro running fio with 4KB random reads at queue depth 32, switching from mq-deadline to none typically drops p99 latency from ~180μs to ~110μs and lifts IOPS from ~620K to ~900K. The scheduler was adding a lock-protected tree operation to a workload where reordering had zero benefit.

Rule of thumb for scheduler selection:

• NVMe: none — the device is faster than the scheduler.
• SATA SSD: mq-deadline — some merging still helps, seek cost is zero.
• HDD: bfq or mq-deadline — seek reordering is the whole game.
• Desktop with mixed workloads: bfq — its per-cgroup fairness prevents one process from starving your UI.

Modern kernels detect NVMe and default to none automatically, but virtualized environments and older distros often still ship mq-deadline as the default. Check with cat /sys/block/*/queue/scheduler — the choice in brackets is active.