IO schedulers

There are 2 queues, one for read & one for write operations.

• none is just a First In First Out standard queue of I/O operations (it still merges some requests).
• cfq (Completely Fair Scheduling) is similar to the Round Robin algorithm and basically allots a fixed execution time for each I/O operation (they are implemented as a circular queue)
• deadline is like a priority queue with an aging concept. Basically it adds a deadline for each I/O operation & implements a priority queue

Supported in kernel >=4.12. It is disabled by default. To use multi-queue schedulers compile kernel with CONFIG_SCSI_MQ_DEFAULT=y or pass parameter scsi_mod.use_blk_mq=1 in boot loader.

Linux Multi-Queue Block IO Queueing Mechanism

For older systems:

/etc/udev/rules.d/60-schedulers.rules

ACTION=="add|change", KERNEL=="sd*[!0-9]|sr*", ATTR{queue/rotational}=="1", ATTR{queue/scheduler}="cfq"
ACTION=="add|change", KERNEL=="sd*[!0-9]|sr*|mmcblk[0-9]*", ATTR{queue/rotational}=="0", ATTR{queue/scheduler}="deadline"  

/etc/udev/rules.d/60-schedulers.rules

ACTION=="add|change", KERNEL=="sd*[!0-9]|sr*", ATTR{queue/rotational}=="1", ATTR{queue/scheduler}="bfq"

# order matters - strict_guarantee:=1 cause slice_idle back to 8
ACTION=="add|change", KERNEL=="sd*[!0-9]|sr*", ATTR{queue/rotational}=="1", ATTR{queue/iosched/strict_guarantees}="1"
ACTION=="add|change", KERNEL=="sd*[!0-9]|sr*", ATTR{queue/rotational}=="1", ATTR{queue/iosched/slice_idle}="0"


ACTION=="add|change", KERNEL=="sd*[!0-9]|sr*|mmcblk[0-9]*", ATTR{queue/rotational}=="0", ATTR{queue/scheduler}="kyber"
ACTION=="add|change", KERNEL=="nvme[0-9]n[0-9]*", ATTR{queue/scheduler}="kyber"

Check current schedulers:

cat /sys/block/sd*/queue/scheduler

/etc/udev/rules.d/60-schedulers.rules

ACTION=="add|change", KERNEL=="sd[a-z]", ATTR{queue/rotational}=="1", ATTR{queue/scheduler}="bfq"
ACTION=="add|change", KERNEL=="sd[a-z]", ATTR{queue/rotational}=="0", ATTR{queue/scheduler}="mq-deadline"

sudo udevadm control --reload
sudo udevadm trigger --action=change --subsystem-match=block
cat /sys/block/sd*/queue/scheduler
cat /sys/block/sd*/queue/iosched/slice_idle
cat /sys/block/sd*/queue/iosched/strict_guarantees

• bfq & btrfs
• bfq