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21.4 Why Both /tmp and /usr/tmp?

[UNIX traditionally has two places to put temporary files: /tmp and /usr/tmp . This article explains how that started. -JP]

As I understand it, the reason for the /tmp - /usr/tmp split is identical to the reason for the /bin - /usr/bin and /lib - /usr/lib splits and is a historical accident of hardware configuration at the Research system.

At one time (circa the time of the original UNIX paper in CACM), the Research machine was a PDP 11/45 with a fixed-head disk, some RK05s, and an RP03. The root went on the fixed-head disk, since the absence of seek times made it fast. But fixed-head disks (anybody remember them?) were tiny . Two megabytes [sic] was a big fixed-head disk. So you had to be fairly careful to avoid overflowing the root filesystem (which included /tmp -it wasn't a separate filesystem). /usr , on the other hand, was the main filesystem on the 40-MB RP03.

So you had a very sharp split of hardware: things directly under / , like /tmp , /bin , and /lib , were fast but had to be small; things under /usr could be big but accesses to them were slower. So you put the heavily used commands in /bin , the heavily used libraries in /lib , and [flourish of trumpets] the small temporary files in /tmp . All the other slush went under /usr , including a /usr/tmp directory for big temporaries. This is why a few programs like sort ( 36.1 ) put their temporaries in /usr/tmp : they expect them to be big. [Though most /usr filesystems are fast these days, a lot of systems still have much more room on /usr/tmp than /tmp . -JP  ]

In practice, fixed-head disks are historical relics now, and much of the justification for the various /x - /usr/x splits has disappeared. There is one reason why you might retain a /tmp - /usr/tmp split, however. If your /tmp filesystem is kept in "RAM disk" or something similar for speed, you might want to keep your editor temp files somewhere else if your editor has crash recovery ( 30.24 ) . Crash recovery definitely works better when the files it is looking for are kept in nonvolatile memory!

- HS in net.unix on Usenet, 19 March 1984


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