Multiple Partitions on OS X - Pros and Cons

Whether to have multiple partitions or not has been a topic of debate for ages... You might think it even predates the invention of storage.

Here, I'll give a rundown of the many reasons why you might want (or not) to have multiple partitions on your OS X hard disk drives.

However, it's important to remember that partitioning is as personal as a computer task can get.

Why? Because every person's usage pattern are unique; Different applications, different storage needs, etc. You get the picture.

As for how many partitions, and what size is right for you, well, only you can reply to these questions.

As with all things in life, there are Pros and Cons... Let's go over them.

Pro 1: Easier backups and management

Keeping the OS separate from your personal data, makes backups easier.
"Cloning" or making an "image" of the OS and Applications is a great way to have your system up-and running, with your specific configuration and tweaks, should something go wrong.
With the huge size of current hard disks, it's impractical to clone very big partitions to optical media.

Also, think about the frequency at which the OS and the Applications you use change.
Now compare that to your iTunes library, your email folders, and your work files.
It's clear those don't change at the same frequency.
Losing a week's worth of OS and application updates, configuration and tweaks is, in most cases, not the same as losing a week's worth of changes on the files you work on.

Pro 2: Reinstall OS without touching your data

If you ever want (or actually, "When you will need") to reinstall your OS (for example, at the next major update), your data can remain untouched.
This doesn't mean you shouldn't back it up... But it will save you from having to restore it.

Pro 3: Improved performance - Less fragmentation

A small OS partition that rarely changes will be less prone to fragmentation than, the one with your data which is constantly being modified and overwritten.
And, while OS X automatically defragments files, there are some limitations to it, like it will only do this on files smaller than 20MB.

Pro 4: Improved performance - Hot File Clustering

As Hard Disk Drives rotate at a constant speed (See: CAV - Constant Angular Velocity), regardless of the position of the Read/Write heads, higher throughput can be achieved from the outer tracks than the inner ones.

Remember how it was much funnier standing on the outer edge of the Carousel than riding the horses near the center ?

Yes, you can thank CAV for that too.

So, keeping your OS on the outer tracks will lead to faster application launches and your system will feel snappier.

The effects of this are so dramatic that it was implemented in HFS+ as an optimization scheme for boot volumes, and it's called "Hot File Clustering".
Under this scheme, the most used ("hottest") files are dynamically selected, defragmented and moved to the "Hot Zone" (the 0.5% outer tracks) of the boot volume.



However, there are some disadvantages too...

Con 1: Requires planning

This is one of the biggest disadvantages.
We are all too eager to get our system up and running and don't want to spend time planning such a seemingly "insignificant" thing.
Or not?

Con 2: Requires understanding your usage patterns

OK, so you've decided you want to plan...
Now this requires gathering replies to some questions:

• Which applications do you use?
• How big is their installation footprint?
• Will it be enough to have just "OS" and "Users"?
• Or it might be wise to have also an "Archive" for seldomly modified files (where performance is less important, so we can put it in the "inner" tracks)?
• May be a "Media" volume for music, photos and movies?

As you can see, there are many questions which require some careful consideration.

Con 3: Requires anticipating your Applications needs

Now, this is another tough one.
Even if you do understand your usage patterns, how can you anticipate the storage requirements of the applications you use?
Now, and in the future?
What about the next version of iWhatever?
Will it be distributed on a zillion DVDs and require 400TB?

Let me tell you that there's no answer to these questions...

So, how can you deal with it? Well, we'll do the best we can.
I normally try to estimate the current needs, and factor a 50% to 100% growth.
This should help keeping the file system "not too full" (which degrades performance), and give me enough time to use my system, without having to repartition...

Con 4: Repartitioning is annoying

Yes, doing it right the first time is better than having to backup, go back to the drawing board, repartition and restore.
However, and no matter how good your planning is, you'll eventually hit the strict limit of one of your partitions.

Con 5: There's free space but not on the "right" volume

Accept it, at one point or another, you'll hit the brick wall, with one of your volumes full, and plenty of free space on other ones.
And, yes... This is irritating too.





Having said all this, take in mind that in some cases, you still might want to dedicate a whole drive for a specific purpose.
Good candidates for this might be your digital music library, a Final Cut Scratch disk, an iMovie raw footage repository, etc.


I'll later post my partitioning scheme, for the curious among you...

So, what's your stand on this?
Do the advantages outweigh the disadvantages? What works best for you? Why?