Chapter 18. 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: Set the scheduler using TuneD , as described in Setting the disk scheduler using TuneD Set the scheduler using udev , as described in Setting the disk scheduler using udev rules Temporarily change the scheduler on a running system, as described in Temporarily setting a scheduler for a specific disk Note In Red Hat Enterprise Linux 8, 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. The traditional, single-queue schedulers, which were available in Red Hat Enterprise Linux 7 and earlier versions, have been removed. 18.1. Available disk schedulers The following multi-queue disk schedulers are supported in Red Hat Enterprise Linux 8: 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 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. 18.2. Different disk schedulers for different use cases Depending on the task that your system performs, the following disk schedulers are recommended as a baseline prior to any analysis and tuning tasks: Table 18.1. Disk schedulers for different use cases Use case Disk 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 . 18.3. The 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, Red Hat recommends to use udev rules or the TuneD application to configure it. Match the selected devices and switch the scheduler only for those devices. 18.4. Determining the active disk scheduler This procedure determines which disk scheduler is currently active on a given block device. Procedure Read the content of the /sys/block/ device /queue/scheduler file: In the file name, replace device with the block device name, for example sdc . The active scheduler is listed in square brackets ( [ ] ). 18.5. Setting the disk scheduler using TuneD This procedure creates and enables 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 selected-scheduler with the disk scheduler that you want to set for the device, for example bfq Prerequisites The TuneD service is installed and enabled. For details, see Installing and enabling TuneD . Procedure 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 . To see which profile is currently active, use: Create a new directory to hold your TuneD profile: Find the system unique identifier of the selected block device: Note The command in the 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 unique ID . Create the /etc/tuned/ my-profile /tuned.conf configuration file. In the file, set the following options: Optional: Include an existing profile: Set the selected disk scheduler for the device that matches the WWN identifier: 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: Enable your profile: Verification Verify that the TuneD profile is active and applied: Read the contents of the /sys/block/ device /queue/scheduler file: In the file name, replace device with the block device name, for example sdc . The active scheduler is listed in square brackets ( [] ). Additional resources Customizing TuneD profiles . 18.6. Setting the disk scheduler using udev rules This procedure sets a given disk scheduler for specific block devices using 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 selected-scheduler with the disk scheduler that you want to set for the device, for example bfq Procedure Find the system unique identifier of the block device: Note The command in the 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 unique ID . Configure the udev rule. Create the /etc/udev/rules.d/99-scheduler.rules file with the following content: 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 ). Reload udev rules: Apply the scheduler configuration: Verification Verify the active scheduler: 18.7. Temporarily setting a scheduler for a specific disk This procedure sets 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: In the file name, replace device with the block device name, for example sdc . Verification Verify that the scheduler is active on the device:

Commands:
cat /sys/block/ device /queue/scheduler [mq-deadline] kyber bfq none
tuned-adm active
mkdir /etc/tuned/ my-profile
udevadm info --query=property --name=/dev/ device | grep -E '(WWN|SERIAL)' ID_WWN= 0x5002538d00000000_ ID_SERIAL= Generic-_SD_MMC_20120501030900000-0:0 ID_SERIAL_SHORT= 20120501030900000
[main] include= existing-profile
[disk] devices_udev_regex= IDNAME = device system unique id elevator= selected-scheduler
devices_udev_regex=(ID_WWN= 0x5002538d00000000 )|(ID_WWN= 0x1234567800000000 )
tuned-adm profile my-profile
tuned-adm active Current active profile: my-profile
tuned-adm verify Verification succeeded, current system settings match the preset profile. See TuneD log file ('/var/log/tuned/tuned.log') for details.
cat /sys/block/ device /queue/scheduler [mq-deadline] kyber bfq none
udevadm info --name=/dev/ device | grep -E '(WWN|SERIAL)' E: ID_WWN= 0x5002538d00000000 E: ID_SERIAL= Generic-_SD_MMC_20120501030900000-0:0 E: ID_SERIAL_SHORT= 20120501030900000
ACTION=="add|change", SUBSYSTEM=="block", ENV{ IDNAME }==" device system unique id ", ATTR{queue/scheduler}=" selected-scheduler "
udevadm control --reload-rules
udevadm trigger --type=devices --action=change
cat /sys/block/ device /queue/scheduler
echo selected-scheduler > /sys/block/ device /queue/scheduler
cat /sys/block/ device /queue/scheduler