An important feature within the Linux I/O stack is a part of the kernel block layer called the I/O scheduler. The issue being addressed here is basically this: I/O requests are being more or less continually issued by the kernel (due to apps wanting to perform various file data/code reads and writes); this results in a continuous stream of I/O requests being ultimately received and processed by the block driver(s). The kernel folks know that one of the primary reasons that I/O sucks out performance is that the physical seek of a typical SCSI disk is really slow (compared to silicon speeds; yes, of course, SSDs (solid state devices) are making this a lot more palatable nowadays).

So, if we could use some intelligence to sort the block I/O requests in a way that makes the most sense in terms of the underlying physical medium, it would help performance. Think of an elevator in a building: it uses a sort algorithm, optimally taking people on and dropping them off as it traverses various floors. This is what the OS I/O schedulers essentially try to do; in fact, the first implementation was called Linus's elevator.

Various I/O scheduler algorithms exist (deadline, completely fair queuing (cfq), noop, anticipatory scheduler: these are now considered legacy; the newest as of the time of writing seem to be the mq-deadline and budget fair  queuing (bfq) I/O schedulers, with bfq looking very promising for heavy or light I/O workloads (bfq is a recent addition, kernel version 4.16). The I/O schedulers present within your Linux OS are a kernel feature; you can check which they are and which is being used; see it being done here on my Ubuntu 18.04 x86_64 box:

$ cat /sys/block/sda/queue/scheduler 
noop deadline [cfq] 
$ 

Here, bfq is the I/O scheduler being used on my Fedora 28 system (with a more recent kernel):

$ cat /sys/block/sda/queue/scheduler 
mq-deadline [bfq] none
$ 

The default I/O scheduler here is bfq. Here's the interesting bit: the user can actually select between I/O schedulers, run their I/O stress workloads and/or benchmarks, and see which one yields the maximum benefit! How? To select the I/O scheduler at boot time, pass along a kernel parameter (via the bootloader, typically GRUB on an x86-based laptop, desktop or server system, U-Boot on an embedded Linux); the parameter in question is passed as elevator=<iosched-name>; for example, to set the I/O scheduler to noop (useful for systems with SSDs perhaps), pass the parameter to the kernel as elevator=noop. 

There's an easier way to change the I/O scheduler immediately at runtime; just echo(1) the one you want into the pseudo-file; for example, to change the I/O scheduler to mq-deadline,do the following:

# echo mq-deadline > /sys/block/sda/queue/scheduler 
# cat /sys/block/sda/queue/scheduler 
[mq-deadline] bfq none
# 

Now, you can (stress) test your I/O workloads on different I/O schedulers, thus deciding upon which yields the optimal performance for your workload.