person Tute World Team schedule Updated April 28, 2026

I/O Scheduler

When your system has dozens of processes all reading and writing to disk at once, who goes first? That's the I/O scheduler's job — it decides the order of disk requests to maximize throughput, minimize latency, and be fair to all processes.

Why Does Order Matter?

Why not just process disk requests in the order they arrive? On spinning hard drives (HDDs), the read/write head physically moves across a spinning platter. If requests arrive in random order, the head spends most of its time seeking back and forth — wasting time. Reordering requests so the head sweeps in one direction (like an elevator) dramatically reduces seek time. On SSDs, there's no moving head, but scheduling still helps with request merging and fairness.

HDD vs SSD — Different Needs

	HDD	NVMe SSD
Seek time	5–15ms (physical head movement)	~0.1ms (no moving parts)
Random vs sequential	Sequential much faster	Comparable speed
Scheduling benefit	High — reorder saves seek time	Low — hardware handles it
Best scheduler	BFQ or mq-deadline	none (no scheduler)
Queue depth	1 (single queue)	Up to 65535 (multi-queue)

The Schedulers — Old and New

none / noop — No Scheduling

No reordering. Requests processed in arrival order (FIFO), with basic merging of adjacent requests. Best for NVMe SSDs where the device's own hardware queue is smarter than any software scheduler. Also used in VMs (the host handles scheduling).

mq-deadline — Deadline-Based

Each request gets a deadline (default: 500ms reads, 5s writes). The scheduler batches requests for throughput but guarantees no request waits past its deadline. Good balance for SSDs and HDDs. Prevents write starvation — a common problem when reads always jump the queue.

BFQ — Budget Fair Queuing

Assigns each process a "budget" of disk sectors. Processes get fair slices of disk bandwidth, similar to CPU scheduling. Great for desktop use: your music player won't stutter while a large file copies. Slightly higher CPU overhead than mq-deadline. Best for HDDs and SATA SSDs on interactive systems.

CFQ — Completely Fair Queuing (legacy)

The old default before multi-queue block layer. Replaced by BFQ. Uses per-process I/O queues with time slices. Still available but deprecated — use BFQ instead.

Check and Change Schedulers

# Check current scheduler for a device
cat /sys/block/sda/queue/scheduler
# mq-deadline [bfq] none   ← brackets = current

cat /sys/block/nvme0n1/queue/scheduler
# [none]   ← NVMe defaults to none

# List available schedulers
cat /sys/block/sda/queue/scheduler
# mq-deadline [bfq] none

# Change at runtime (takes effect immediately)
echo mq-deadline > /sys/block/sda/queue/scheduler
echo none > /sys/block/nvme0n1/queue/scheduler

# Make permanent (udev rule)
# /etc/udev/rules.d/60-scheduler.rules:
ACTION=="add|change", KERNEL=="sd[a-z]", ATTR{queue/scheduler}="bfq"
ACTION=="add|change", KERNEL=="nvme[0-9]*", ATTR{queue/scheduler}="none"

Which Scheduler to Use?

Workload	Device	Recommended	Why
NVMe SSD (any workload)	NVMe	none	Device handles queuing natively
Desktop / interactive	HDD/SATA SSD	BFQ	Fair per-process, no stutter
Database server	SATA SSD	mq-deadline	Lower latency, deadline guarantees
VM guest	Virtual disk	none	Host hypervisor handles it
Spinning HDD, latency matters	HDD	mq-deadline	Prevents starvation

Tuning Queue Depth and Read-Ahead

# Queue depth — how many requests device can handle at once
cat /sys/block/sda/queue/nr_requests
# 64  ← default, good for most cases

# Read-ahead — pre-fetch sequential data (in 512-byte sectors)
cat /sys/block/sda/queue/read_ahead_kb
# 128   ← default

# Increase read-ahead for sequential workloads (large file reads, video)
echo 2048 > /sys/block/sda/queue/read_ahead_kb

# Disable for random-access workloads (databases with their own cache)
echo 0 > /sys/block/sda/queue/read_ahead_kb

# Check I/O stats per device
cat /proc/diskstats
# or
iostat -x 1   # requires sysstat package

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