M1 Max internal memory bandwidth related to external ssd speed

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Level 1

4 points

M1 Max internal memory bandwidth related to external ssd speed

I apologize up front for sharing my confusion. I just know that I don't understand this issue fully, and I hope someone smarter than I can help educate me, please?

I have a Mac Studio, M1 Max, 32 GPU cores, 64 GB RAM. If I understand the relationships between the various devices, I think that the 400 GB/s bandwidth of the internal RAM is the top capacity that can move through the CPU. That's my starting assumption.

Here's more background (as far as I understand it) before I get to my actual questions: I have a couple of external SSDs connected via the Thunderbolt 4 ports. These are NVMe drives in Thunderbolt 4 certified enclosures. If I understand correctly, I think these can top out around 32 GB/s, and that fits just fine within the Thunderbolt 3/4 spec of 40 GB/s. And that is nowhere near the 400 GB/s speed of the RAM. Right so far?

I'm interested in the OWC miniStack STX. (https://eshop.macsales.com/shop/owc-ministack-stx). It has two purposes: (1) Act as a hub to add three more powered Thunderbolt 4 ports, and (2) provide room for two internal drives. I'm interested in both purposes. It connects to a Mac via a Thunderbolt 4 connection = 40 GB/s. That makes sense to me for the three additional Thunderbolt 4 ports = mission accomplished.

The specs on the miniStack state that you can get up to 700 MB/s "data transfer speed". I can't imagine that applies to the additional Thunderbolt 4 ports. I can only understand the 700 MB/s speed as applying to the internal drives.

Here are my questions:

Is the 700 MB/s data transfer speed the same thing as bandwidth?
Why would I want/need a device that can only provide 700 MB/s data transfer speed from the miniStack's internal drives (one of which can be an NVMe drive) via a Thunderbolt 4 connection?
The fact that it has a Thunderbolt 4 connection can only really apply to the hub aspect of the miniStack, yes?
The Thunderbolt 4 connection from the miniStack is terribly underused when transferring data at a rate of 700 MB/s from the miniStack's internal drives, right?
Since my external NVMe SSDs can transfer approx 32 GB/s, I'm better off keeping those connected via the Thunderbolt 4 connections on my Mac and using the miniStack as a hub, yes? In this case, I can get much cheaper Thunderbolt 4 hubs for that purpose than getting the miniStack? I mean, I guess I could put a mechanical hard drive into the miniStack and call that my Time Machine, but that just seems to be a "solution" looking to solve a problem that isn't really a problem?

I'm sorry for the lengthy post. I wanted to lay out my logic to see if anyone can correct me or help me understand it better. I'm sure I can't be understanding all of the implications, so I wanted to run this by smarter people before I make a purchase that kind of steps me backwards from what I already have in place.

Thanks in advance!

Posted on Jan 8, 2023 12:22 PM

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Answer 1

Jan 8, 2023 3:08 PM in response to Jeff Ring

This stuff is really complicated, and everyone is expressing speeds using different units and arbitrarily using bits vs Bytes vs Giga-Transfers/sec. You should not be red-faced, YOU had the nerve to look inside.

You may have jumped back too far.

The fastest SSD drive available can run in a PCIE 3 slot NVME card at about 3500 M Bytes/sec.

The fastest Thunderbolt enclosure can support that same fast single SSD drive at about 2500 M Bytes/sec, after overhead.

ThunderBolt-3 allows up to 4 lanes of PCI Express 3.0 (32.4 G bit/s) for general-purpose data transfer. [that's 3.200 G Bytes/sec, or 3,200 M Bytes/sec.] Today that top transfer speed might require simultaneously reading and writing from multiple drives in a Thunderbolt enclosure, likely sold as a RAID enclosure.

A really fast Rotating magnetic drive can source or sink a Burst at up to about 150 M Bytes/sec, but generally can't do that fast steady state.

a SATA-III 6G drive connects at up to 600 M Bytes/sec.

The OWC enclosure you are looking at is an interesting compromise. I don't think it is the fastest at anything.

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Answer 2

Jeff Ring Author

Level 1

4 points

Jan 9, 2023 1:21 PM in response to Grant Bennet-Alder

Thank you for unpacking that more. I really appreciate it!

The miniStack is absolutely a compromise — it’s a hub, and it’s an external storage device, but it’s not the best at either of those things… and for my purposes, it doesn’t need to be. I just wanted to make sure I wasn’t creating my own bottleneck by not considering capacity and bandwidth (✅✅). For my purposes, it’s a reasonable compromise, as I won’t be going backwards in capacity or bandwidth. And going forward, I’ll know to be more discerning when reading specs. 🙂👍

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Answer 3

Jan 8, 2023 1:06 PM in response to Jeff Ring

it's all in the upper/lower case of one letter.

the fastest single SSD readily available today can transfer sequential big blocks at MOST 3,500 M Bytes/sec. You reported speeds 10 time faster, possibly reading bits as Bytes, related by a factor of about 10.

That speed might be attainable on a internal PCIe4 bus.

Single SSD drives on Thunderbolt busses are limited to about 2500 M Bytes/sec transfer speeds.

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Answer 4

Jeff Ring Author

Level 1

4 points

Jan 8, 2023 2:42 PM in response to Grant Bennet-Alder

Oh, my word. I like to think I'm smarter than that, but I think you nailed exactly my mistake. Thank you for your speedy response and diagnosis of where I went wrong. Boy, is my face red!

With that mistake ironed out, I think that fixes my misunderstanding and answers my question about the 700 MB/s transfer speed. Given that my external SSD is moving at 32 MB/s, I'm way off in my concern about 700 MB/s being slower. 700 MB/s is plenty fast for what I want. (Am I correct in that understanding now?)

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M1 Max internal memory bandwidth related to external ssd speed

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