SSTable is an on-disk file format

File format

A file format is a particular way that information is encoded for storage in a computer file.Since a disk drive, or indeed any computer storage, can store only bits, the computer must have some way of converting information to 0s and 1s and vice-versa. There are different kinds of formats for...

 that represents a string-to-string mapping

Map (mathematics)

In most of mathematics and in some related technical fields, the term mapping, usually shortened to map, is either a synonym for function, or denotes a particular kind of function which is important in that branch, or denotes something conceptually similar to a function.In graph theory, a map is a...

. It is an immutable

Immutable object

In object-oriented and functional programming, an immutable object is an object whose state cannot be modified after it is created. This is in contrast to a mutable object, which can be modified after it is created...

 system so that once written, the map remains unchanged.

The file system stores key-value pairs that are sorted by the key. The write is done sequentially with an index at the end of the file. By creating the index, it suffices to keep the index in memory for efficient lookups. In order to load the table for random access

Random access

In computer science, random access is the ability to access an element at an arbitrary position in a sequence in equal time, independent of sequence size. The position is arbitrary in the sense that it is unpredictable, thus the use of the term "random" in "random access"...

into memory, a process will first seek the index, find the link to the data and read only necessary information from the disk.