The Linux Kernel May Soon Default To SCSI MQ Mode
Written by Michael Larabel in Linux Storage on 3 July 2018 at 04:28 AM EDT. 8 Comments
LINUX STORAGE --
It looks like the Linux kernel's SCSI code may soon switch away from its legacy code-path to the multi-queue (MQ) code by default.

The SCSI multi-queue code has been in the works for a few years as enabling the block multi-queue (blk-mq) code for SCSI drivers for better scalability. The SCSI MQ support has been part of the mainline kernel for a few years now but has been disabled by default that requires either changing the kernel configuration or booting with the scsi_mod.use_blk_mq=Y parameter.

Those unfamiliar with the SCSI MQ code can see this older presentation (PDF). SCSI MQ, like the blk-mq code, is successful on delivering better performance/scalability, but it hadn't been enabled by default due to some outstanding regressions.

But SUSE's Johannes Thumshirn is now looking to enable the multi-queue code by default for SCSI over the legacy I/O path. He commented, "It has been more than one year since we tried to change the default from legacy to multi queue in SCSI. Back then we had to retract the change because of performance issues with rotating disks. In the meantime there have been a substantial amount of performance improvements, thus we can re-enable scsi-mq without a significant performance penalty."

So far those commenting on the patch have all been support in the change, including developers from both Red Hat and SUSE. We'll see if this change ends up happening for the Linux 4.19 cycle.
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Michael Larabel is the principal author of Phoronix.com and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 10,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated benchmarking software. He can be followed via Twitter or contacted via MichaelLarabel.com.

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