Linux Buffer Caching

Reading from a disk is very slow compared to accessing (real) memory. In addition, it is common to read the same part of a disk several times during relatively short periods of time. By reading the information from disk only once and then keeping it in memory until no longer needed, one can speed up all but the first read. This is called disk buffering , and the memory used for the purpose is called the buffer cache .

Since memory is, unfortunately, a finite, nay, scarce resource, the buffer cache usually cannot be big enough (it can't hold all the data one ever wants to use). When the cache fills up, the data that has been unused for the longest time is discarded and the memory thus freed is used for the new data.

Disk buffering works for writes as well. On the one hand, data that is written is often soon read again, so putting data that is written in the cache is a good idea. On the other hand, by only putting the data into the cache, not writing it to disk at once, the program that writes runs quicker. The writes can then be done in the background, without slowing down the other programs.

Most operating systems have buffer caches , but not all of them work according to the above principles. Some are write-through : the data is written to disk at once. The cache is called write-back if the writes are done at a later time. Write-back is more efficient than write-through, but also a bit more prone to errors: if the machine crashes, or the power is cut at a bad moment, or the floppy is removed from the disk drive before the data in the cache waiting to be written gets written, the changes in the cache are usually lost.

The cache does not actually buffer files, but blocks, which are the smallest units of disk I/O (under Linux, they are usually 1 KB). This way, also directories, super blocks, other filesystem bookkeeping data, and non-filesystem disks are cached.

The effectiveness of a cache is primarily decided by its size. A small cache is next to useless: it will hold so little data that all cached data is flushed from the cache before it is reused. The critical size depends on how much data is read and written, and how often the same data is accessed. The only way to know is to experiment.

The Linux command “free” will list used memory and memory used for buffer caching