# 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
```