Skip to main content

Disk Performance

====================================================================================

I/O Schedulers

In Linux, the I/O scheduler is responsible for determining the order in which block I/O operations are submitted to storage devices. The scheduler affects the performance and behavior of disk I/O operations, impacting both throughput and latency. Different I/O schedulers are available, each optimized for specific workloads and scenarios.

By default, I/O tasks are scheduled by fifo - first in first out.

Common Linux I/O Schedulers

  1. CFQ (Completely Fair Queuing) - Provides a balanced approach to I/O scheduling, aiming to give each process a fair share of the I/O bandwidth.
  2. Deadline - Designed to prevent starvation of I/O operations by imposing deadlines on requests.
  3. NOOP - Implements a simple FIFO (First-In, First-Out) queue, essentially a passthrough scheduler.
  4. BFQ (Budget Fair Queuing) - Aims to provide predictable I/O performance by distributing I/O bandwidth according to budgets assigned to tasks.
  5. MQ Deadline (Multiqueue Deadline) - Similar to the Deadline scheduler but designed for multiqueue block devices.
  6. Kyber - A relatively new scheduler designed to work well with modern hardware and to provide low latency.
  7. BFQ (Budget Fair Queuing) -  Bypasses software I/O scheduling, relying entirely on hardware-level I/O management.

Changing the scheduler

The scheduler is set on a per disk basis, not per filesystem.

(none persistent)  method:

The scheduler can be set within the below file:

/sys/block/diskname/queue/scheduler

Within this file, you'll likely see a number of the potential I/O scheduler types. Tne one surrounded by [] is the currently selected scheduler, ie:

cat /sys/block/sda/queue/scheduler
[mq-deadline] none
(persistent) method:

To persistently set a disk scheduler, we'll need to alter the grub configuration

/etc/default/grub

append the below to the line prefaced with 'GRUB_CMDLINE_LINUX='

elevator=schedulertype

Save the new GRUB configuration:

update-grub2

or

grub-mkconfig

====================================================================================

Disk Performance Troubleshooting Tools

------------------------------------------------------------------------------------------------------------------------------------------------

sar

------------------------------------------------------------------------------------------------------------------------------------------------

lsof

We can use the lsof command to check what parts of a disk a process is accessing.

lsof -p pid

lsof -c command

------------------------------------------------------------------------------------------------------------------------------------------------

systat tools

The sysstat package includes a collection of performance monitoring tools for Unix-like systems.

iotop

iotop [options]
-a aggregate disk IO over time (duration of command)
iostat

Similar to iotop. Provides statistical information about I/O device loading. It reports on CPU utilization, device I/O statistics, and system throughput, making it useful for overall system performance analysis.

iostat
ioping

ioping is basically the ping command but for disks. It is used to measure the IO latency of storage devices - basically it measures how long a storage device will take to respond to an I/O request.

ioping [options] target
-c specify a number of IO requests to make
-i interval between requests
-s request size (default 4kb)
-q suppress regular output, only show statistics

====================================================================================

No Comments
Back to top