Catalina - volumes vs partitions in practice - a question or two

So, with the two standard Catalina volumes residing in a single Container, and therefore sharing the free space in the container, as one adds more to the physical disk over time (eg. macOS updates, more apps, more data) doesn't the actual data written to disk become mixed across the physical disk regardless of which volume it belongs to?

Yes. But so what. The advantage of Containers is that they dynamically allocate storage as necessary, within the volume in which those Containers reside.

It doesn't help that Apple publishes very little publicly accessible APFS documentation. You already read more about it than just about anyone else.

With partitions, on the other hand, if you create a MacHD partition for macOS + apps and a second, separate partition for data there is no shared space between them, ever.

Not all the time, and that's a problem. The primary reason for separating the read-only Macintosh HD from your data is to make macOS invulnerable to malicious alteration, as that Support document explains in Apple's characteristically vague language.

My advice: there is no physical separation anyway, so stop trying to think about it in those terms. Volumes remain what they have always been: no physical separation either, so unless you have a need to format a particular volume for a required file structure (Windows perhaps) then don't partition a disk.

In case you did not read this from Partition a physical disk in Disk Utility on Mac,

"However, with APFS, you shouldn’t partition your disk in most cases. Instead, create multiple APFS volumes within a single partition. With the flexible space management provided by APFS, you can even install another version of macOS on an APFS volume."

And pay attention to the Important note that immediately follows (if you're interested).

If I understand correctly, in a standard installation of Catalina there is a single Container that holds two Volumes -

There are five. There is also a Preboot, VM, and Recovery.

I don't quite understand how the MacHD volume remains "completely separate from all other data" if it shares the free space with the MacHD-Data volume and the space is allocated to each of them as needed.

You have files stored in folders. The files are actually stored all over the drive, even in separate pieces, mixed in with other files. But somehow, they are still in a single folder. Not a perfect analogy, but I imagine it is similar.

What might I be missing about the "separateness" that is claimed between the two Catalina volumes?

That there is virtual separateness.

Really, a partition is virtual separateness. The files in different partitions are still all on the same drive. How are they separate, but still in the same physical medium?

It is separate, but attempting to ascribe physical separation as humans accept in the physical world is a fool's errand. Interleaving is another antiquated concept that no longer exists in any semblance that may have been comparable to magnetic media storage. APFS abandoned any previously accepted comparison to "files" or "folders" as their etymology might suggest, when the actual means of accomplishing data storage shouldn't be so limited.

An iPhone user who grew up in this century generally doesn't know or care what a "file" is. And forget about volumes. They just want their magical devices to work their magic. It's rather a problem when they expect literal magic from them (and they do), but that's the demographic Apple created.

Apple is trying to oversimplify things by describing the Macintosh HD volume as completely separate from any other. It just isn't. Suffer a catastrophic SSD failure and you'll know.

Partitioning creates physically separate areas on the disk. Files and folders in a partition exist only in the physical space of that partition. They are not stored "all over the drive" (although they may be stored "all over the partition").

Whether they cross some virtual boundary or not, they are on the same physical space. How is the partition boundary created. Is it fixed? How does the partitioning software divide the physical disk?

Why does it matter?

I may be guilty of having overlooked any questions regarding spinning hard disks. As far as Apple is concerned they might as well already be dead. I couldn't agree more.

APFS was designed specifically for solid state storage, to both exploit its advantages and obviate magnetic media's disadvantages. Of course APFS works with hard disks too, but most of its benefits are lost on them.

MartinR wrote:

Whether they cross some virtual boundary or not, they are on the same physical space. How is the partition boundary created. Is it fixed? How does the partitioning software divide the physical disk?

(I'm only answering questions here, I'm not preaching.)

I wasn't actually asking those questions. I just provided them as thought exercises.

The first partition, which is typically the boot partition, always resides at the "beginning" of the disk - the outermost tracks - and is the fastest responding part of the drive. It has been my practice to restrict the first partition to OS X/macOS + apps thereby maximizing system performance.

The OS has done this for at least a decade. You don't need to do it yourself.

With APFS being optimized for solid state storage, this may not happen in APFS.