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Chapter 3. Setting the disk scheduler

The disk scheduler is responsible for ordering the I/O requests submitted to a storage device. You can configure the scheduler in several different ways:

Note

In Red Hat Enterprise Linux, block devices support only multi-queue scheduling. This enables the block layer performance to scale well with fast solid-state drives (SSDs) and multi-core systems.

3.1. Available disk schedulers
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Choosing the appropriate disk scheduler significantly impacts system performance, responsiveness, and the efficiency of I/O operations. Understanding the role and characteristics of each available scheduler enables users to select the most suitable option for their specific workload and hardware. The following multi-queue disk schedulers are supported in Red Hat Enterprise Linux:

none
Implements a first-in first-out (FIFO) scheduling algorithm. It merges requests at the generic block layer through a simple last-hit cache.
mq-deadline

Attempts to provide a guaranteed latency for requests from the point at which requests reach the scheduler. The mq-deadline scheduler sorts queued I/O requests into a read or write batch and then schedules them for execution in increasing logical block addressing (LBA) order. By default, read batches take precedence over write batches, because applications are more likely to block on read I/O operations. After mq-deadline processes a batch, it checks how long write operations have been starved of processor time and schedules the next read or write batch as appropriate.

This scheduler is suitable for most use cases, but particularly those in which the write operations are mostly asynchronous.

bfq

Targets desktop systems and interactive tasks. The bfq scheduler ensures that a single application is never using all of the bandwidth. In effect, the storage device is always as responsive as if it was idle. In its default configuration, bfq focuses on delivering the lowest latency rather than achieving the maximum throughput.

bfq is based on cfq code. It does not grant the disk to each process for a fixed time slice but assigns a budget measured in the number of sectors to the process. This scheduler is suitable while copying large files and the system does not become unresponsive in this case.

kyber
The scheduler tunes itself to achieve a latency goal by calculating the latencies of every I/O request submitted to the block I/O layer. You can configure the target latencies for read, in the case of cache-misses, and synchronous write requests. This scheduler is suitable for fast devices, for example NVMe, SSD, or other low latency devices.
Default disk scheduler
Block devices use the default disk scheduler unless you specify another scheduler.
Note

For non-volatile Memory Express (NVMe) block devices specifically, the default scheduler is none and Red Hat recommends not changing this.

The kernel selects a default disk scheduler based on the type of device. The automatically selected scheduler is typically the optimal setting. If you require a different scheduler, use udev rules or the TuneD application to configure it. Match the selected devices and switch the scheduler only for those devices.

Depending on the task that your system performs, use the following disk schedulers as a baseline prior to any analysis and tuning tasks:

Expand
Use caseDisk scheduler

Traditional HDD with a SCSI interface

Use mq-deadline or bfq.

High-performance SSD or a CPU-bound system with fast storage

Use none, especially when running enterprise applications. Alternatively, use kyber.

Desktop or interactive tasks

Use bfq.

Virtual guest

Use mq-deadline. With a host bus adapter (HBA) driver that is multi-queue capable, use none.

3.3. Determining the active disk scheduler
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You can view the disk scheduler that is currently active on a given block device.

Procedure

  • Read the content of the /sys/block/device/queue/scheduler file: 

    # cat /sys/block/device/queue/scheduler
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    [mq-deadline] kyber bfq none
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    In the file name, replace device with the block device name, for example sdc. The active scheduler is listed in square brackets ([ ]).

3.4. Setting the disk scheduler using TuneD
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You can create and enable a TuneD profile that sets a given disk scheduler for selected block devices. The setting persists across system reboots. In the following commands and configuration, replace:

  • device with the name of the block device, for example sdf and
  • selected-scheduler with the disk scheduler that you want to set for the device, for example bfq.

Prerequisites

Procedure

  1. Optional: Select an existing TuneD profile on which your profile will be based. For a list of available profiles, see TuneD profiles distributed with RHEL.

  2. To see which profile is currently active, use: 

    # tuned-adm active
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  3. Create a new directory to hold your TuneD profile: 

    # mkdir /etc/tuned/<new-profile>
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  4. Find the system unique identifier of the selected block device: 

    # udevadm info --query=property --name=/dev/device | grep -E '(WWN|SERIAL)'
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    ID_WWN=0x5002538d00000000_
ID_SERIAL=Generic-_SD_MMC_20120501030900000-0:0
ID_SERIAL_SHORT=20120501030900000
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    Note

    The command in this example will return all values identified as a World Wide Name (WWN) or serial number associated with the specified block device. Although it is preferred to use a WWN, the WWN is not always available for a given device and any values returned by the example command are acceptable to use as the device system’s unique ID.

  5. Create the /etc/tuned/my-profile/tuned.conf configuration file. In the file, set the following options:

    1. Optional: Include an existing profile: 

      [main]
include=<existing-profile>
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    2. Set the selected disk scheduler for the device that matches the WWN identifier: 

      [disk]
devices_udev_regex=IDNAME=device system unique id
elevator=selected-scheduler
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      Here:

      • Replace IDNAME with the name of the identifier being used (for example, ID_WWN).

      • Replace device system unique id with the value of the chosen identifier (for example, 0x5002538d00000000). To match multiple devices in the devices_udev_regex option, enclose the identifiers in parentheses and separate them with vertical bars: 

        devices_udev_regex=(ID_WWN=0x5002538d00000000)|(ID_WWN=0x1234567800000000)
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  6. Enable your profile: 

    # tuned-adm profile <new-profile>
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Verification

  1. Verify that the TuneD profile is active and applied: 

    $ tuned-adm active
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    Current active profile: <profile>
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    $ tuned-adm verify
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    Verification succeeded, current system settings match the preset profile.
See TuneD log file ('/var/log/tuned/tuned.log') for details.
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  2. Read the contents of the /sys/block/device/queue/scheduler file: 

    $ cat /sys/block/device/queue/scheduler
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    [mq-deadline] kyber bfq none
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    In the file name, replace device with the block device name, for example sdc. The active scheduler is listed in square brackets ([]).

3.5. Setting the disk scheduler using udev rules
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You can set a given disk scheduler for specific block devices by using the udev rules. The setting persists across system reboots. In the following commands and configuration, replace:

  • device with the name of the block device, for example sdf and
  • selected-scheduler with the disk scheduler that you want to set for the device, for example bfq.

Procedure

  1. Find the system unique identifier of the block device: 

    # $ udevadm info --name=/dev/device | grep -E '(WWN|SERIAL)'
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    E: ID_WWN=0x5002538d00000000
E: ID_SERIAL=Generic-_SD_MMC_20120501030900000-0:0
E: ID_SERIAL_SHORT=20120501030900000
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    Note

    The command in this example will return all values identified as a World Wide Name (WWN) or serial number associated with the specified block device. Although it is preferred to use a WWN, the WWN is not always available for a given device and any values returned by the example command are acceptable to use as the device system’s unique ID.

  2. To configure the udev rule, create the /etc/udev/rules.d/99-scheduler.rules file with the following content: 

    ACTION=="add|change", SUBSYSTEM=="block", ENV{IDNAME}=="device system unique id", ATTR{queue/scheduler}="selected-scheduler"
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    Here:

    • Replace IDNAME with the name of the identifier being used (for example, ID_WWN).
    • Replace device system unique id with the value of the chosen identifier (for example, 0x5002538d00000000).

  3. Reload udev rules: 

    # udevadm control --reload-rules
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  4. Apply the scheduler configuration: 

    # udevadm trigger --type=devices --action=change
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Verification

  • Verify the active scheduler: 

    # cat /sys/block/device/queue/scheduler
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You can set a given disk scheduler for specific block devices. The setting does not persist across system reboots.

Procedure

  • Write the name of the selected scheduler to the /sys/block/device/queue/scheduler file: 

    # echo selected-scheduler > /sys/block/device/queue/scheduler
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    In the file name, replace device with the block device name, for example sdc.

Verification

  • Verify that the scheduler is active on the device. 

    # cat /sys/block/device/queue/scheduler
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