/tmp and /usr/tmp

[Henry Spencer]

Brian is close.  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 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 file system (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(1) put their temporaries in /usr/tmp:
they expect them to be big.

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 tempfiles somewhere else if your editor
has crash recovery.  Crash recovery definitely works better when the files
it is looking for are kept in non-volatile memory!
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry