What does it mean when Linux has no I/O scheduler

I have some virtual machines running Ubuntu cloud-based image 14.04-1 LTS version. I wanted to see the IO performance of different IO schedulers on the VM so I went to /sys/block/<drive>/queue/scheduler on the guest OS to change the IO scheduler. Usually, there should be cfq, deadline, or noop to choose. But what I saw is none. Does it mean that Canonical has removed the I/O scheduler in the cloud-based image or the scheduler none here is the renamed noop scheduler? and what happens if we don't have an I/O scheduler in the system? All the io requests were directly sent the host in FIFO order?

Thanks for shed some light!

Solution 1:

From this Debian Wiki:

Low-Latency IO-Scheduler

(This step is not necessary for SSDs using the NVMe protocol instead of SATA, which bypass the traditional I/O scheduler and use the blk-mq module instead.)

The default I/O scheduler queues data to minimize seeks on HDDs, which is not necessary for SSDs. Thus, use the "deadline" scheduler that just ensures bulk transactions won't slow down small transactions: Install sysfsutils and

echo "block/sdX/queue/scheduler = deadline" >> /etc/sysfs.conf

(adjust sdX to match your SSD) reboot or

echo deadline > /sys/block/sdX/queue/scheduler

So, the answer is: none is NOT an alias for noop. none means "the scheduler is not used".

Solution 2:

It seems that on kernels >= 3.13 none is not an alias of noop anymore. It is shown when the blk-mq I/O framework is in use; this means a complete bypass of the old schedulers, as blk-mq has (right now) no schedulers at all to select.

On earlier kernels, none really is a poorly-documented alias for noop. See here for more details.

Solution 3:

None is not an alias for noop.

None is displayed because no scheduler is in use. SSDs using the NVMe protocol instead of SATA bypass the traditional I/O scheduler.

Solution 4:

Guest VMs have virtual I/O devices provided by the hypervisor. Therefore the actual I/O device scheduling is performed by the hypervisor kernel, and guests pass all device I/O directly to hypervisor without any scheduling.