DASD - block level scheduler set to none

This document (000019894) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Linux Enterprise Server 15 SP2
SUSE Linux Enterprise Server 12 SP5

Situation

The situation applies to DASD (Direct access storage device) on s390x.

On SLES12 SP5 the multiqueue series of schedulers was introduced and for dasd disks the default scheduler changed from 'deadline' to 'none'.

The nr_requests (number of requests which can be queued up at blocklevel) also decreased from 128 to 32.
This and the scheduler set to 'none' can have an effect on performance, especially throughput in sequential reads.

Resolution

The blk-mq scheduler sets nr_requests to the configured queue depth (see blk_mq_init_allocated_queue in block/blk-mq.c), For DASD the default is currently 32.
If a higher value is needed it can be changed with:

# echo VALUE > /sys/block/<device>/queue/nr_requests

A patch to change or restored the default scheduler to mq-deadline is available and a PTF can be requested from SUSE Customer Service.

SLES12 SP5, the s390-tools update (s390-tools-2.1.0-18.26.1)
and SLES15 SP2 the s390-tools update (s390-tools-2.11.0-9.20.1)
both released March 2021 include the patch.

Disclaimer

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