Because it used to be necessary.
Early VM systems *required* at least one page of swap for every page of
physical ram. In theory, the entire contents of physical ram would be
copied at any time onto swap, and whenever it was necessary to free up a
page to bring in a new one, it would already be on backing store and could
easily be freed up.
This was prior to things like being able to page in binaries from file
systems on demand, so your programs had to be swapped back out as well.
So, basically, your totally usable paging area was the sum of swap space.
Not the sum of swap space + physical memory.
Now, granted, this is no longer the case for most (all?) VM based systems,
the rule of thumb has such strong momentum behind it that it's difficult to
Now, personally, I tend to throw on a few meg of swap onto each disk and
stripe swap across them for performance. If I find I ever run out of
memory under normal use, I'll up it. But I've never had that happen. Swap
space depends on how you use it. Set up some stuff, and monitor with free
every once in a while. If you never hit swap, then reduce it or eliminate
it. If you are constantly running over 1/3 of it or so, might consider
upping it a little bit.
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