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Linux Multi-Queue Block IO Queueing Mechanism (blk-mq)

blk-mq (Multi-Queue Block IO Queueing Mechanism) is a new framework for the Linux block layer that was introduced with Linux Kernel 3.13 and has become feature-complete with Kernel 3.16.
Blk-mq allows for over 15 million IOPS with high-performance flash devices (e.g. PCIe SSDs) on multi-socket servers, though even single and dual socket servers also benefit considerably from blk-mq.
To use a device with blk-mq, the device must support the respective driver.

Overview of blk-mq

Blk-mq integrates into the storage stack and provides basic functions to device drivers for mapping I/O enquiries to multiple queues.
The tasks are distributed across multiple threads and therefore to multiple CPU cores (per-core software queues).
Blk-mq compatible drivers request count for parallel hardware queues a device supports.
All device drivers that use the previous block I/O layer continue to work independently of blk-mq.

blk-mq-based device drivers bypass the previous Linux I/O scheduler.

Device Drivers

Driver Device Name Supported Devices
null_blk /dev/nullb* none (test drivers)
virtio-blk /dev/vd* Virtual guest drivers
mtip32xx /dev/rssd* Micron RealSSD PCIe
scsi (scsi_mq) /dev/sd* e.g. SAS and SATA SSDs/HDDs
NVMe /dev/nvme* e.g. Intel SSD DC P3600 DC P3700 Series
rbd /dev/rdb* RADOS Block Device (Ceph)
ubi/block /dev/ubiblock*
loop /dev/loop* Loopback-Device
dm / dm-mpath

Enable blk-mq

Enabling blk-mq must happen at boot time. You need to add scsi_mod.use_blk_mq=1 to your lilo.conf

# LILO configuration file
# generated by 'liloconfig'
# Start LILO global section
# Append any additional kernel parameters:
append=" scsi_mod.use_blk_mq=1"
Enabling blk_mq makes it impossible to use non blk_mq schedulers. You lose noop cfq and the non mq deadline

Automatic IO scheduler selection

udev rules can be used to select IO schedulers for different device types.

# set deadline scheduler for non-rotating disks
ACTION=="add|change", KERNEL=="sd[a-z]", TEST!="queue/rotational", ATTR{queue/scheduler}="deadline"
ACTION=="add|change", KERNEL=="sd[a-z]", ATTR{queue/rotational}=="0", ATTR{queue/scheduler}="bfq"
# set cfq scheduler for rotating disks
ACTION=="add|change", KERNEL=="sd[a-z]", ATTR{queue/rotational}=="1", ATTR{queue/scheduler}="cfq"


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