====== 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.
<note>
**blk-mq-based** device drivers bypass the previous Linux I/O scheduler.
</note>


====== 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__

<file bash /etc/lilo.conf>
# LILO configuration file
# generated by 'liloconfig'
#
# Start LILO global section
# Append any additional kernel parameters:
append=" scsi_mod.use_blk_mq=1"

</file>

<note>
Enabling __blk_mq__ makes it impossible to use non __blk_mq__ schedulers. You lose __noop cfq__ and the non mq __deadline__
</note>

====== Automatic IO scheduler selection ======
 udev rules can be used to select IO schedulers for different device types.

<file bash /etc/udev/rules.d/60-ioscheduler.rules>
# 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"
</file>

====== Sources ======
 * Originally written by [[wiki:user:lamerix | lamerix]] \\
 * Source [[https://www.thomas-krenn.com/en/wiki/Linux_Multi-Queue_Block_IO_Queueing_Mechanism_(blk-mq)|https://www.thomas-krenn.com/en/wiki/Linux_Multi-Queue_Block_IO_Queueing_Mechanism_(blk-mq)]] \\
 * Source [[http://kernel.dk/blk-mq.pdf|http://kernel.dk/blk-mq.pdf]] \\
 * Source [[https://lwn.net/Articles/552904/|https://lwn.net/Articles/552904/]] \\

{{tag>howtos blk-mq multi-queue io author_lamerix}}