File systems have been a standard part of computer systems for over 50 years. We routinely (and mostly unthinkingly) use them to store and retrieve persistent data. But how persistent are they really?
This question is far trickier to answer than it may seem at first, for several reasons. Persistent storage media are orders of magnitude slower than non-persistent memories, such as processor registers or random-access memory. They are also organized differently: persistent media are read or written in blocks whose size is on the order of kilobytes but can be up to megabytes. Bridging the speed and granularity gaps requires complex logic for data buffering, implementing efficient lookups, and reordering slow operations to obtain the best performance. Some of this is performed by the operating system, and some of it in the firmware of the storage device, with limited operating-system control.
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