What is the default I/O scheduler in RHEL8?

Environment

• Red Hat Enterprise Linux 8

Issue

• Need to know the default I/O scheduler in RHEL 8

Resolution

• Available I/O schedulers in RHEL8 are mq-deadline, kyber, bfq, and none.

  # cat /sys/block/sda/queue/scheduler 
  [mq-deadline] kyber bfq none
  

• No specific I/O scheduler is assigned in RHEL8. Instead, the scheduler is chosen on a by-device basis depending on its type.

  • The scheduler can be set using How to persistently set the I/O scheduler.

IO Scheduler Selection Issue in 8.3 and 8.4

If default IO scheduler selection issues are noticed, then ensure you are running at RHEL 8.5, or later, in order to have a patch that corrects default io scheduler selections.
 

A bug was introduced somewhere between RHEL 8.3 and 8.4 kernels (specifically within kernel 4.18.0-252.el8). The wrong io scheduler may end up being selected. Some customers may see their scheduler changing when upgrading from RHEL 8.3 to RHEL 8.4 environments without indication as to why.

RHEL8.2: (!)4.18.0-193.el8           27-Mar-2020   ok

4.18.0-235.el8                         ok

RHEL8.3: (!)4.18.0-240.el8           23-Sep-2020   ok

4.18.0-252.el8                         bad commit 8856c9842ca8 added from upstream

4.18.0-277.el8                         bad

RHEL8.4: (!)4.18.0-305.el8           29-Apr-2021   bad

4.18.0-311.el8                         fix added via commit cf6c57ea8d29

RHEL8.5: (!)4.18.0-348.el8           04-Oct-2021   fix included

RHEL8.6: (!)4.18.0-372.9.1.el8       15-Apr-2022   fix included

:

.

8856c9842ca8 blk-mq: Facilitate a shared sbitmap per tagset (broke default io scheduler selection in some cases)

cf6c57ea8d29 blk-mq: set default elevator as deadline in case of hctx shared tagset (fix for above)

The fix was released via Errata RHSA-2021:4356 in kernel-4.18.0-348.el8 (RHEL 8.5) and is present in 8.5 and later kernels.

Root Cause

• Details on scheduler selection can be seen in the official documentation: 15.3. 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.
  

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