To Partition or...! Virtual Memory

[Charles Martin]

> From: Richard McKay <richard.mckay1 at virgin.net>
>
> Charles Martin wrote the following:
>
>> Yes. OS X has an integrated virtual memory that CANNOT be turned off.
>> This is a Good Thing.
>
> Just out of curiosity...has anyone tried /know what happens when there 
> is
> only the absolute minimum RAM (taken removable RAM out) and filled (no 
> doubt
> this would have to be done on purpose and then done a defragmentation 
> and
> filled again to ensure total disk space usage) the drive...then ran 
> whatever
> programs would start and tried switching or using large items copied 
> to the
> clipboard? As VM is never off, what happens?

An excellent question that I happen to be able to answer, as I've 
recently discovered what happens.

First of all, no modern Mac will boot if all the RAM is removed. I 
know, kind of obvious, but I wanted to cover all bases.

Next, if there is not sufficient RAM (128MB), you may get a message 
that Mac OS X does not want to install itself because there is not 
enough RAM. Once OS X is installed, of course, you don't get this 
message. I have run Mac OS X (as an experiment) on a machine (G3/500 
iMac) with as little as 32MB of actual RAM. Boy did that run slowly!

Jaguar, you may be interested to know, actually ran kind of OK on the 
G3/500 with 64MB. I mean, it was slow, but quite usable if you stuck to 
one app open at a time, particular small, lithe apps like Safari. :)

But to answer your real question ("what happens when the RAM *and* the 
disk are full?") the answer is that OS X automatically reserves at 
least 100MB for itself for just this purpose. The system will report 
that the disk is full even when there is that much space available, and 
no new non-temp files can be written to it. Thus, the system always 
reserves an 80MB "block" of VM for itself, and thus can continue 
operating (although it will of course run REALLY slowly).

Note how much better that is than the way OS 9 handles "RAM full" or 
"disk full" situations. :)

> Is this documented somewhere?

As far as I know, there's only this: 
http://docs.info.apple.com/article.html?artnum=107334
(refers to Classic being unable to start if there isn't sufficient 
space left for IT'S vm file)

The rest is based only on my personal experience.

_Chas_

"Here is the thing you will learn from really using an OS X Macintosh, 
and must somehow accept on faith if that's what it takes to get you to 
Switch: Apple makes design decisions based on a sincere desire to make 
your life better." -- Glenn McDonald