CompTIA Linux+ XK0-005 - 4.1 - I/O Scheduler

In Linux, the I/O scheduler is a component of the kernel that determines the order in which input/output (I/O) requests are serviced. The I/O scheduler plays a crucial role in optimizing disk performance and improving overall system responsiveness. This guide will provide an overview of I/O schedulers, including common ones like none, mq-deadline, kyber, and bfq, and explain how to change the I/O scheduler at runtime and persistently using udev rules and GRUB configuration.

What is an I/O Scheduler?

An I/O scheduler is responsible for managing I/O operations between the CPU and storage devices, such as hard drives and solid-state drives (SSDs). It helps prioritize and schedule the order in which read and write requests are serviced, aiming to reduce latency and maximize throughput.

Common I/O Schedulers

The Linux kernel provides several I/O schedulers. Here are some commonly used schedulers:

none: This scheduler executes I/O requests in the order they are received without any specific prioritization or optimization.
mq-deadline: It is a multiqueue variant of the deadline scheduler that provides fairness and deadline-based prioritization for I/O operations.
kyber: Designed to optimize disk I/O performance for both SSDs and rotational drives, kyber focuses on low-latency and responsiveness.
bfq: Known as Budget Fair Queueing, bfq offers low-latency and fairness by assigning budgets to processes for disk access.

Changing the I/O Scheduler at Runtime

You can change the I/O scheduler dynamically during system runtime. Follow these steps:

Open a terminal on your Linux system.
Check the available I/O schedulers for a specific disk device using the following command:
```
cat /sys/block/{device_name}/queue/scheduler
```
Identify the current active scheduler indicated by square brackets [ ].
Change the I/O scheduler by writing the desired scheduler name to the appropriate sysfs file using the following command:
```
echo {scheduler_name} > /sys/block/{device_name}/queue/scheduler
```
Replace {scheduler_name} with the desired scheduler and {device_name} with the appropriate disk device (e.g., sda, nvme0n1).
Example
```
echo none > /sys/block/sda/queue/scheduler
```
Verify that the I/O scheduler has been changed by running the cat command from step 2 again.

Changing the I/O Scheduler Persistently

Info

It's recommended to consult the official documentation and relevant resources specific to your Linux distribution for detailed instructions and any potential differences. For example, Ubuntu has started using udev rules over grub for the i/o scheduler settings.

Using udev Rules

To change the I/O scheduler persistently using udev rules, follow these steps:

Open a terminal on your Linux system.
Create a new udev rule file using a text editor. For example:
```
vi /etc/udev/rules.d/60-schedulers.rules
```
Add the following line to the file to set the desired I/O scheduler for all disk devices:
```
ACTION=="add|change", KERNEL=="sd[a-z]", ATTR{queue/scheduler}="{scheduler_name}"
```
Replace {scheduler_name} with the desired scheduler (e.g., none, mq-deadline, kyber, bfq).
Example
```
ACTION=="add|change", KERNEL=="sd[a-z]", ATTR{queue/scheduler}="none"
```
Save the file and exit the text editor.
Trigger the udev rules to apply the changes without rebooting:
```
udevadm trigger
```

Using GRUB Configuration

To change the I/O scheduler persistently using GRUB configuration, follow these steps:

Open a terminal on your Linux system.
Edit the GRUB configuration file using a text editor. For example:
```
vi /etc/default/grub
```
Find the line starting with GRUB_CMDLINE_LINUX and append the following parameter:
```
elevator={scheduler_name}
```
Replace {scheduler_name} with the desired scheduler (e.g., none, mq-deadline, kyber, bfq).
Example
```
GRUB_CMDLINE_LINUX="crashkernel=auto resume=/dev/mapper/os_vg-swap rd.lvm.lv=os_vg/root rd.lvm.lv=os_vg/swap rhgb quiet elevator=none"
```
Save the file and exit the text editor.

Update GRUB configuration to apply the changes:

On Ubuntu and Debian-based systems, run:
```
update-grub
```

On Red Hat and CentOS-based systems, run:

# BIOS systems
grub2-mkconfig -o /boot/grub2/grub.cfg

# EFI systems
grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg

Reboot your system for the changes to take effect.

Conclusion

Understanding the concept of I/O schedulers and how to change them is essential for optimizing storage performance and troubleshooting storage-related issues. By selecting an appropriate I/O scheduler, you can improve I/O throughput, reduce latency, and enhance overall system responsiveness. Remember to consider the characteristics and requirements of your workload when choosing an I/O scheduler.

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