If it's a boot hard drive and your interested in maintaining the computers maximum read/write performance, no more than the first 50% of the drive filled.
Meaning no files or anything on the second 50% of the boot hard drive, this can be done by creating a second partition in Disk Uitlity, provided there is ample room. If there isn't, then there is something there already and you have to reduce items so only 50% of the boot hard drive is filled, then do a defrag via my method here to shift the data up to the first 50% of the boot hard drive.
You can still get decent performance from a boot hard drive until it's about 80% filled, however in later OS X versions there are a lot of large hidden caches like versions, timemachine local backups (laptops) and such that will unfortunatly push your newly created or transferred files to the more slower parts of the boot hard drive.
If you have a boot SSD then don't let it get more than 80% filled.
However other things can make a computer slow,
Ramón G Castañeda wrote:
Interesting. I had always thought that partitioning the boot drive actually negates the benefits of having plenty of available space on the boot volume itself.
Partitioning of the boot drive does not negate the availability of space on the boot drive, it keeps OS X background crap from growing too large, updates, program installs and/or the user from writting files to the slower second 50% of the boot hard drive thus maximizing read/write performance at all times.
The second partition doesn't have to go unused, it can be used to store files that the user is unconcerned if the performance begins to suffer with items written there.
For instance I have my boot hard drive 50/50 with B being a bootable clone of A.
The boot time of B is remarkably slower as well as other performance "feel" issues while booted into that B partition.
However since its intention is a alternate boot/back up system, I'm not concerned about it's performance, rather that it works when I need it.
Thank you for the detailed clarification, ds store. What you write makes sense, course
Alas, I'm afraid I phrased my previous thought unartfully. My bad, sorry.
What I should have written instead is something like:
I had always thought that partitioning the boot drive actually negates the VM benefits of having plenty of available space on the boot volume itself, i.e., in regards to the available space being used by the OS for its swap files (VM) and—if you have only one drive, as is usually the case in a laptop—and shared by the scratch file of applications like resource-hog Photoshop when there's no additional, physically separate, dedicated, internal or external hard drive to designate as its primary scratch disk.
Again my apologies for the imprecision of my previous post.
Needless to say, I wouldn't even think of launching Photoshop without a physically separate, dedicated, hard drive; but sometimes circumstances can force one to do so. Sometimes I work with very large files (well over 1 GB), and I have on occasion seen Photoshop's scratch exceed 160 GB or more.