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

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.

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".

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.