Why does my M3 MacBook Air internal SSD have 3 containers?

DeFaygo wrote:

Why does my internal SSD have 3 containers?

I don't believe it does have 3 containers – just one that Disk Utility describes as "Container disk3". This is an APFS Container whose associated Unix device is /dev/disk3 ("disk3" for short).

What are "Container disk1," and "Container disk2?"

I see no such containers in your screenshot. The device names disk0 – disk2 probably were already taken. There might be partitions that even Disk Utility does not list, where things like firmware are stored.

What is "Container disk3?" Is it the same as "Macintosh HD?"

An APFS container allows several volumes or volume groups to share a pool of free disk space. Instead of dividing up a 2 TB drive as 500 GB for FILES and 1.5 TB for PHOTOS, you can have separate FILES and PHOTOS volumes – while still allowing the FILES to take up more than 500 GB of space, if needed.

Apple is taking advantage of that APFS container feature here.

What you see as "Macintosh HD" in the Finder is a combination of everything in the "Macintosh HD' volume group that resides within the APFS container "disk3." Apple broke "Macintosh HD" into pieces in the name of increased security, and the Finder is hiding the true complexity of the startup disk from you.

• The nested "Macintosh HD" is a cryptographically sealed volume that contains key system files. It is sealed so that if malware tries to modify it, modifications will be detected. Only Apple installers and Apple tools have the ability to modify this thing in a way that will leave a correct signature.
• The "Macintosh HD snapshot" is a further defense against malware. Instead of directly using the sealed system volume, the Mac runs off a read-only "snapshot" of it.
• The "Data" volume is where anything on "Macintosh HD" that the system needs to be able to modify (in normal operation) goes. This includes some system files as well as all of your data.

Unfortunately, one side effect of this is to make it a lot harder to make bootable clone backups. You win some, you lose some …

What is "Macintosh HD snapshot?"Can I get rid of "Macintosh HD snapshot?"

Part of the hardening of the macOS installation against malware attack. You can't get rid of it.

@Servants of Cats has done a good job explaining the drive layout, but it is a bit more complicated since macOS is hiding some details of the drive layout from users these days. Those hidden items on an M-series Macs are required system items to allow the M-series Macs to even boot into the Startup Options screen. Those hidden items used to be part of the system firmware on older Macs. For this reason, if the internal SSD fails, the M-series Macs will be unable to even boot to an external SSD unlike older Intel Macs.

Technically the M-series Macs' internal SSD does have three APFS Container. It is just coincidence that those APFS Containers are listed as "disk1", "disk2", and "disk3". The "diskX" designations are device identifiers and are assigned on system boot and when external devices are connected. A lot of things can cause these device identifiers to be different on each boot.

Here is the drive layout of my M1 laptop running Big Sur showing those other hidden system APFS Containers:

/dev/disk0 (internal):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                         500.3 GB   disk0
   1:             Apple_APFS_ISC ⁨⁩                        524.3 MB   disk0s1
   2:                 Apple_APFS ⁨Container disk3⁩         494.4 GB   disk0s2
   3:        Apple_APFS_Recovery ⁨⁩                        5.4 GB     disk0s3

Notice that "disk1" and "disk2" are not shown. "disk1" & "disk2" would have been used during system boot when the two hidden system APFS Containers (disk0s1 & disk0s3) were mounted & used. Once the system was booted, those two hidden system Containers are no longer needed so they were unmounted releasing the "disk1" & "disk2" device identifiers.

As for "Purgeable" storage that is storage that is still being used by macOS, but can be released at some unknown time in the future. Ignore the "Available" storage value since it is very misleading. It is unfortunate that Apple decided to show the "Available" value everywhere within macOS since it consists of "Purgeable" space which is still being used by the system (to some extent anyway since that storage has not been released yet). The most important storage value is the "Free" space value since that is the amount of storage that can be used immediately. The Free space value is only explicitly shown in the Apple System Profiler and within Disk Utility.

FYI, you need to make sure you always have at least 20GB+ of Free storage space at all times for the normal operation of macOS. Some workloads will require having even more Free space...perhaps even over 100GB+ especially if editing HD videos. 20GB is the absolute bare minimum since even that 20GB can quickly disappear even for moderate tasks. If an APFS volume completely runs out of Free storage space, then you won't even be able to delete any data to make more space due to how the APFS file system works which would then require you to start completely over by erasing the disk followed by reinstalling macOS & restoring from a backup.

DeFaygo wrote:

If my internal SSD is 245.11 GB, why does it say that 190.53 GB is used with 132.13 GB available? (322.66GB)

What is the "purgeable" 96.15 GB?

It probably means that there are files taking up space on the SSD that the system can – and will – delete if it needs the space. Say, caches, including caches of files that you are storing on iCloud Drive.

I would not recommend installing any "cleaner" applications as some of those may try to remove files that should be left alone.

DeFaygo wrote:

Thank you for your reply. You've been very informative and helpful. In the screenshot there is a colored bar. The colored bar is divided into three sections. "Container disk1" is very thin, it is blue, and it is on the left side, and it is 524.3 MB. "Macintosh HD" is very big, it is red, and it is in the middle, and it is 245.11 GB. "Container disk2" is fairly small, it is orange, and it is on the right side and it is 5.37 GB. There are 3 colored chips below the colored bar that say "Container disk1," "Macintosh HD," and "Container disk2" indicating their size.

I see those two now – on my system as well, with the same exact sizes. I'm guessing that Disk Utility does not show them in the list in the sidebar on purpose.

In the colored bar, Macintosh HD might represent the entire volume group "Macintosh HD volumes" - the things that make up what the Finder displays as "Macintosh HD".