The most volumes that I ever used on a single drive was about six. Back when the Mac’s file system didn’t cope particularly well with crashes or huge numbers of files, that was not unusual. I had a boot volume, two for my music library so that classical and modern tracks didn’t have to compete for the same space, a couple of working areas in which I coded and debugged my apps, and a bit of spare space for anything else.
Over the last fifteen years or so, most of us have taken advantage of improving robustness in the HFS+ file system and the progressive awkwardness of multiple volumes. I now can’t remember the last time that I formatted my Mac’s internal drive into more than one partition.
Apple’s new file system, APFS, coming later this year in macOS High Sierra, will change that.
In the past, volumes or partitions have occupied an odd place in the structure of our storage. At the top, both HFS+ and APFS deal with fixed-size drives, which correspond to their physical equivalent. Some file systems can span a collection of storage devices, and RAID and Core Storage can achieve the same for HFS+ and APFS: a Fusion Drive consists of two physically separate devices clagged together by software, and a four-drive RAID accomplishes much the same.
But these are software illusions that are achieved by magic outside the file system itself. For the time being, if your Time Machine backup drive is running short of free space, you can’t bolt on an additional external drive and use it to accommodate the overflow.
Folders are at the opposite end: they can grow and shrink in size as much as you like, and are easily created and removed. In return for that flexibility, they are weak containers for relatively limited amounts of content.
Between these extremes are volumes, which are stronger containers but in return have been fixed in size. Recent versions of OS X have been able to perform non-destructive adjustments to the partitioning of a drive, but these are by no means guaranteed. The more you have stored on a drive, and the more radical the intended alteration to the partition map, the less the chances of it being non-destructive.
The result is that, under HFS+ at present, volumes often make it harder to manage your storage. Many people have partitioned the drive used for their Time Machine backups, so that they can store other files on a separate volume on that drive, only to regret it when they have wanted to increase the size of the backup volume.
APFS changes this with its completely different concept of volumes, and should make them of much greater use to Mac users.
In HFS+, volumes are allocated in fixed sizes from the space on the drive. If you are partitioning a 1 TB drive, you can have two 500 GB drives, but together they account for the entire drive space, so that is all you can get.
In APFS, volumes only occupy the physical space that they actually need. You could have four or more 500 GB volumes on that 1 TB drive. The limit is imposed by the data actually stored in each: that cannot of course exceed the total capacity of the drive (although because APFS also use sparse storage, that can happen in effect). So if your four 500 GB volumes only have 100 GB stored in each, there is still a total of 600 GB free space for those four volumes.
Sparse storage ensures that storage space is only used to store data, not empty content, and has long been available in disk images. If you have a 1 GB file most of which is empty, then APFS stores the data that it needs to, and not the whole 1 GB. So in practice that very large file could only occupy a few MB of your storage.
In High Sierra, when using APFS, it makes sense to create a new volume whenever its contents would benefit from a more robust container than a mere folder. If you want to store backups and other files on the same drive, create a volume for each, and they can grow to fill the space that they need, as they need it. You don’t have to worry about trying to change the partition map, as APFS effectively does that for you, as it goes along. So long as the total used space remains below the total capacity of the drive, APFS will provide free space to keep your backups running properly.
I don’t think it will be long before my record number of volumes will be beaten into the dust.