That's the oversimplified view in practice I'm sure there's a lot of complications and fiddly details that need to be dealt with. So what you can do is build a new-format database-ish thing in the free space in the volume (copying and converting the contents of the old-format one), and then simply say "ok, use the new one now." (and mark the old one as free space in the new format). That means that in converting from one format to another, most of the volume's contents doesn't need to change only the database-ish thing that stores file locations and metadata needs to be converted. Most of the disk's contents is (generally) the raw file contents, since the metadata is generally much smaller than the file's actual contents.
The raw file contents is pretty much the same between different volumes formats (except for encrypted volumes), but the database-ish metadata will be different between HFS+ vs. Somewhat oversimplified, there are two kinds of data being stored: the actual file contents are just stored pretty much as raw data, and then there's some additional database-ish thing that keeps track, for each file, of where that particular file's raw data is stored (as well as "metadata" like file's name, permissions, last modify time, etc). To understand what that means, you need to know a little bit about how a filesystem stores its files on disk. I don't know much in terms of details, but I'm pretty sure the basic process involves building parallel volume data structures and then switching which one's used. This article provides some analogies, and I found some hints on how APFS allows easier resizing of partitions.ĭoes the software directly convert one file system to another on the fly? If APFS is basically an enhancement to HFS+ I can see this being easy, but my impression is that is not the case.ĭoes it create two partitions and slowly move content from one to the other, resizing as it goes, or create an image of the old system then restore it to the new system? It seems these would require a fair amount of free space, and could fail if the drive is full. I've done some searches but have only found references to the fact that it can be done. My question is, how, technically, is this accomplished? I'm not asking how a user can do the conversion, I'm asking what actually happens on the disk during the conversion process. This can happen automatically during an OS upgrade or can be done by the user with Disk Utility. Since High Sierra, Apple has allowed users to do an in-place conversion of an existing HFS+ file systems to APFS without losing files.
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