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What exactly happens when I partition an SSD, mounted both, and ran simultaneous disk speed tests?

Example in MacOS:

Multiple partitions in MacOS

I currently have a 500GB SSD in an external enclosure connected by USB. It's split into multiple partitions that backed up different Macbooks (200GB, 200GB, 100GB). All three are bootable, one is encrypted w/ Filevault (and doesn't automount until you type in the password) and the other two are not.

I can access all three partitions simultaneously (I'm also spammed w/ reminders from all 3 partitions, but that's off-topic). This encompasses many different aspects, but what exactly is going on here? I have yet to even mention to internal SSD that's partitioned to run bootcamp and a clean install of Mojave.

Related questions:

What are the drawbacks of doing this? There has a to be hard limits to read/write speed, no? Or else, you could just buy a large SSD and run it in raid 0 by default. It certainly doesn't work this way, does it?

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    No need to apologize. If you're not allowed to post pictures yet you can upload them on a service like imgur (which is also used by SE) and link them. Your subject line says "... mount both" and it's unclear what you're referring to. Also please add the subject to your actual question, they are different things. Maybe check how to ask a good question if you haven't already. Welcome to SU/SE.
    – Seth
    Commented May 23, 2019 at 11:10
  • @seth I accidentally flagged your comment.... you can tell im new here. that was not intentional. also, I'm not sure how I missed the memo w/ imgur links - that makes things much easier. Cheers x2
    – A Potato
    Commented May 23, 2019 at 11:15
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    What exactly happens when I partition an SSD....what exactly is going on here? It's completely unclear what you're asking. You appear to be making some assumptions that you've not shared with us that would explain why you're asking these questions.
    – I say Reinstate Monica
    Commented May 23, 2019 at 11:45
  • I suppose I'll go back to the drawing board and make sure my question is more clear next time. thanks for the feedback. Do I leave this up for now?
    – A Potato
    Commented May 23, 2019 at 11:48
  • Sounds like a question on how the partition is an abstraction over the physical disk. I'll take a stab at it. See also disk partitioning on wikipedia.
    – RJFalconer
    Commented May 23, 2019 at 17:19

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What exactly happens when I partition an SSD, mounted both, and ran simultaneous disk speed tests? ... What exactly is going on here?

It sounds like you're asking about the apparent strangeness of running sped tests against 2 partitions of the same physical disk.

This works, as from the point of view of the testing software they're apparently separate disks*, so the application is just making read/write requests to "both".

Consider when the disk-speed-test sends 1 request to write to the first partition, and then another request to write to the second partition. The second request must wait for the first to finish, so you'd expect your measured performance to be approximately halved.

In actuality the operating system (and disk-controller) is managing the queueing/buffering of writes to the disk addresses and actual SSD sectors. In terms of results, this may mean that you get worse than half performance, as there's now some overhead juggling the two sets of read-write requests.

*Notably some testing tools would perform tests against the physical disk interface, not the partition, so it would not be possible to run two tests against the same physical device. Whatever software you are using to run the test isn't doing this.

Or else, you could just buy a large SSD and run it in raid 0 by default

Raid is only meaningful across a set of disks. Running a single disk in RAID0 is the same as not running it in raid at all (give or take differences in the controller you are using).

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