Mastering Linux I/O Schedulers: CFQ, NOOP, Deadline, and AS Explained

I. Summary of I/O Scheduler

When writing or reading data blocks to/from a device, requests are placed in a queue awaiting completion.

Each block device has its own queue.

The I/O scheduler maintains the order of these queues to use the medium more efficiently, turning unordered I/O operations into ordered ones.

The kernel must first determine the total number of requests in the queue before scheduling.

II. Four I/O Scheduling Algorithms

1) CFQ (Completely Fair Queuing)

Features: CFQ is the default I/O scheduler in recent kernels and distributions, suitable for general servers, multimedia applications, and desktop systems. It distributes I/O bandwidth evenly, avoids starvation, and provides low latency, balancing between deadline and AS schedulers. CFQ assigns each I/O request a priority independent of process priority.

How it works: CFQ creates a separate queue for each process/thread, scheduling these queues using time slices, ensuring each process receives fair I/O bandwidth. The scheduler executes four requests from a process at a time.

2) NOOP (Elevator Scheduler)

Features: Implemented as a simple FIFO queue, it merges new requests with the nearest existing one, suitable for flash devices, RAM, and embedded systems. It tends to starve read requests in favor of writes.

3) Deadline

Features: Classifies requests by time and disk region, similar to NOOP. It guarantees service within a configurable deadline, with a shorter read deadline than write, preventing write starvation. Ideal for database environments (Oracle RAC, MySQL, etc.).

4) AS (Anticipatory Scheduler)

Features: Similar to Deadline but inserts a 6 ms pause after the last read before scheduling other I/O, improving read performance at the cost of some write throughput. Suitable for write‑heavy workloads like file servers, but performs poorly for databases.

III. Viewing and Setting the I/O Scheduler

View current scheduler:

cat /sys/block/sda/queue/scheduler
noop anticipatory deadline [cfq]

Temporarily change scheduler (e.g., to NOOP):

echo noop > /sys/block/sda/queue/scheduler

Permanently change scheduler by adding elevator= to kernel boot parameters (e.g., deadline):

vim /boot/grub/menu.lst
# add: kernel /boot/vmlinuz-... ro root=LABEL=/ elevator=deadline rhgb quiet

After reboot, verify:

cat /sys/block/sda/queue/scheduler
noop anticipatory [deadline] cfq

IV. ionice

ionice adjusts the I/O priority of tasks, but works only with the CFQ scheduler. Examples:

ionice -c1 -n7 -p time dd if=/dev/sda1 of=/tmp/test bs=2M count=300 &
ionice -c2 -n3 -p time dd if=/dev/sda1 of=/tmp/test bs=2M count=300 &
ionice -c3 -n0 -p time dd if=/dev/sda1 of=/tmp/test bs=2M count=300 &

ionice provides three classes: real‑time (highest), best‑effort (default), and idle (lowest). Priorities range from 0 (highest) to 7 (lowest) and are independent of CPU nice values.