Q What is 4K random write speed?

A A measure of how quickly multiple small files can be written to a device.

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4K random write is a disk access pattern whereby small (4K) blocks of data are written to random locations on the surface of a storage device at a queue depth of one. The term is used primarily within the context of benchmarking and the speed which it is usually measured in, MBps, can be thought of as how effective a device is at quickly saving small pieces of information to random locations on the disk. This type of access pattern is common when running applications from a disk and also when copying large application directories. All else being equal drives that are used to host operating systems will perform much better if they are able to sustain high random write speeds. As a rule of thumb approximately 25% of a typical users overall disk access on a PC will consist of random writes.