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I understand the OS needs some free disk space for running. So does windows or any *nix.

But OS X is regarded as a very polished system and my question here is more like, shouldn't it already reserve all disk space that it needs so it won't ever complain (or bug out) about not having enough space?

If it's 10 gb, then get a 64 gb disk and say "you got 44 gb free space, because 20 is used by the system" (supposing OSX already takes 10). Just take the free space needed already! Make a partition if need to be! Hide it if you really want.

Then it wouldn't need to bother the user with worrying about that. There must be a good reason why it's like this, but I couldn't find out. Can you please explain?

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Interesting solution. But we could only propose pros and cons; only Apple could tell you their reasons (and they aren't likely to). –  JRobert Nov 18 '12 at 21:18
    
@JRobert I was actually hoping someone could point to an Apple article where they would tell a general reasoning but not get in to any technical details, like they usually do. –  Cawas Nov 18 '12 at 21:32
    
You as the user already have the tools to accomplish this. Make a second partition on your system and store everything you classify as not system on the second partition. You can grow and shrink that partition on demand and let the system have the balance. You will see that the system doesn't grow it's needs unexpectedly or progressively over time and instead fits in a contained space quite well. –  bmike Nov 19 '12 at 15:54
    
@cawas - I've agreed with the close votes and cast the final vote to close this. Hit us up in Ask Different Chat if you want to edit this to be more of a practical question by getting to the problem you wish to solve. As it stands - it's a nice discussion about OS design but not something that's answerable. I don't think we'll delete this, but I also don't see leaving it open indefinitely for more people to answer unless it's more narrowly scoped... –  bmike Nov 19 '12 at 15:58
    
@bmike I find it weird closing a question that was already mostly answered... but well, can't care about it now! ;-) –  Cawas Nov 20 '12 at 1:02
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2 Answers

up vote 6 down vote accepted

The amount of space the OS needs is neither fixed nor predictable. It depends on a huge number of factors: what programs you run, what you do in them, what background activity they trigger, what background activity happens for other reasons, etc. Hard drive space is used for overflow from RAM, logs, temporary files, etc. If the system reserved a fixed amount of space for this, it would wind up being bigger than it needs to be a lot of the time (and hence waste space), and too small some of the time (which would be very bad).

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Makes a lot of sense... Except for the part where the OS bugs out if you reach zero free space. The OS should protect itself to never bug out, some how. Shouldn't it? –  Cawas Nov 18 '12 at 21:31
    
How? I mean, there are some things it can do, like quitting programs that don't seem to be in active use and thus freeing up the resources they're using, but A) there's a limit to how much can be done this way and B) if there is more space available why should the OS go into panic diet mode just because it's hitting some arbitrary limit? –  Gordon Davisson Nov 18 '12 at 22:56
    
Put it another way: the ultimate reason to not put a limit like this is flexibility. Whenever I've partitioned a computer, I always later find I've chosen the partition sizes poorly: I either need more space on the system partition when there's lots left in the user data partition or vice versa. Not having a fixed dividing line means I can always use all of the available space for whatever it happens to be needed for. –  Gordon Davisson Nov 18 '12 at 22:59
    
You're missing my point. I also advocate to use only 1 partition. I'm not saying "partitioning is the solution, why they don't do it". I'm just saying "how can't they come up with any kind of solution, even if using partitions, to keep the OS working without worrying about free disk space". Windows and Ubuntu sure bug out with zero space. I thought OSX didn't, but now I'm not so sure - not only because of the warning. –  Cawas Nov 18 '12 at 23:06
    
I was just using partitioning to illustrate the more fundamental point that needs change unpredictably, and any other fixed-usage scheme will have at least roughly similar problems. –  Gordon Davisson Nov 18 '12 at 23:09
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Warning messages like these might be a nuisance to some, but are important for most, less savvy users. The purpose is to warn users (who might not know any better) that they are running out of space. As we know, a disk that is too full results in performance lags. Alerting users to this helps to reduce the need for "support" from Apple or other authorized support facilities for issues that are not really related to the OS or hardware malfunction.

It is understandable that repeatedly seeing warning messages like this could be annoying. If so, it may behoove the user to heed them, and move non-critical data to an external drive, freeing up space. Again, it's intended as an alert for those who aren't familiar with how an OS needs to interact with the hardware, and those users constitute the vast majority.

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