I have a Macbook Air with 128GB SSD. It's used to be a best practice to put the OS on a separate partition from data.
Does that make any sense? What are the advantages to partitioning an SSD?
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Unless you have two radically different sets of data such as
I personally see very few reasons to partition your 128GB SSD, and a number of reasons to avoid doing so.
One pressing reason to avoid partitioning a "small" drive: partitions "waste" space. I'm not referring to the actual disk overhead lost to store the partition data, but the loss of usability when a larger contiguous chunk of space is broken into two or more smaller storage areas. If your single partition has 80GB of space available, the largest single file can be 80GB in size. If you partition your 128GB drive to split the empty space, each partition would have 40GB of room, thus cutting the largest file you could save in half.
I use this extreme example to make this point: all partitions end us with some storage space that is "too small" to use. Every partition you add multiplies this loss... And your SSD is not large enough to waste space this way.
Speaking of "small" drives, performance tends to degrade as you fill a partition beyond 75% or 85%. Multiple small partitions tend to reach this percentage filled much faster than a single, larger one.
In the early days of hard drives when the OS was effectively still built around floppy drives, there were real upper-end limits to the number of files or size of the logical drive. Partitions not only made sense then, but were mandatory to shoehorn newer and larger technology into existing systems. Today, those reasons are no longer pressing on our minds.
Even with a MacBook Air, there is no speed advantage to limiting the number of files or size of a partition. Caching and indexing on OS X allow you to "find", retrieve and open any file much faster than on platter-based storage.
In most cases there is no problem housing multiple users on a single OS X machine. Each user is sequestered from the others and provides decent inter-user security and confidentiality.
I suggest you look at your thoughts regarding partitions and utilize folders to replicate the storage functionality. Looking at my Mac Pro with a 64GB boot/application/"user" drive and a separate array with 6TB of storage, I have the following breakdown on the 6TB drive:
I treat each top-level folder as if it was a partition unto itself.
There are no compelling advantages to partitioning the main SSD/HDD for a single OS install under OS X.
Regardless of SSD vs. HDD, it really only makes sense if you need to re-install your OS frequently (which you shouldn't need to do with OS X), and want to be able to keep your data around while wiping your OS partition. There's no performance benefit to be had, and it requires more tinkering to get things set up (symlinking User directories, etc.).
One of the reasons to partition in the past was to improve speeds for some files by taking advantage of the physical location of the data (transfer speeds are higher on the outside of a disk platter). With SSDs, physical location doesn't have an effect on performance, and modern OSes can optimize for best disk performance on HDDs automatically, so there is not much gain to be had from micro managing storage allocation as in the past.
Unless you have a very specific reason for not doing so, just use one partition and enjoy your Mac.
Just a point on scratch drives - SSD's are not recommended for scratch (virtual memory) in 2015 for two reasons (there may be more) - SSD's don't enjoy constant overwrite and data swappping and it will reduce the life of an expensive drive. Using a clean partition set aside on an internal Sata drive such as a WD Black (with 64MB on board cache) will be more than adequate for VM use in programmes such as Photoshop - if you have a Mac Pro tower you can make this even more responsive by installing two identical 1 TB drives and setting them up for Raid - the file is effectively split in two and written to both drives at once to double the speed. I dont set VM on my SSD - I keep it for the OS and primary Admin home folders - that's it.