NVMe SSD in External Enclosure - DRAM Vs HMB?

shake on 16/03/2025 - 19:01
Last edited 16/03/2025 - 19:04

Hi all - looking for some advice from tech gurus.

I'm on the lookout for a suitable NVMe SSD to house in an external enclosure, to use primarily as a working/storage drive to expand the onboard storage on my Mac.

One thing I'm confused about, is the importance of DRAM cache on the SSD. I know that some drives will omit the DRAM cache in order to bring costs down, and instead implement HMB to improve performance.

But what I don't understand is - does HMB actually work when the SSD is connected via an external enclosure (as opposed to being plugged into the motherboard)? Or does it depend on the connection interface in play (eg. USB4)? I've done a bit of searching, but haven't yet found a simple clear explanation on this.

Ultimately I'm trying to work out whether I should be looking at NVMe SSD with DRAM (eg. Kingston KC3000), or whether I can 'make do' with a DRAM-less option (eg. Lexar NM790).

Thanks for any help!

Computing Computer Component M.2 SSD Enclosure SSD

Comments

  • Clear on 16/03/2025 - 20:08
    +4

    HMB won't work with as USB enclosure, so you'll be more relying on the DRAM cache or pSLC cache with the DRAMless SSDs.

    DRAM SSDs will obviously help maintain speed during large transfers, while DRAMless SSDs will drop in speed (often quite significantly) when the pSLC cache is exhausted. Essentially if you plan on doing large sustained transfers, i.e. video editing and backups go for a DRAM SSD, while if you're using it for general usage and more light loads, get a DRAMless SSD. Preferably TLC over QLC.

    • 8bitsperbyte on 16/03/2025 - 22:36
      +1

      HMB will actually work in USB4 and Thunderbolt enclosures - these enclosures are more expensive though. Taking into account the cost difference between a USB 3.x enclosure and USB4/TB enclosure, it might be worthwhile to put the money towards getting an SSD with DRAM instead.

      • shake on 16/03/2025 - 23:25

        Thanks for the tip.

        I've seen mention of HMB being supported over USB4/TB4, but have yet to come across any articles/sources properly describing when it's possible (eg. is a specific controller chipset required?)

        I was leaning towards getting a USB4/TB4 enclosure anyway, to take advantage of the higher transfer speeds of PCIe 4.0 SSDs - so if I can know for certain that HMB will work as a substitute of sorts for onboard DRAM cache, then I'd probably just get a DRAM-less SSD like the Lexar NM790.

        If you know of any links please do share.

        • Clear on 16/03/2025 - 23:29
          +1

          For HMB to work in a USB4/Thunderbolt enclosure, the SSD must have direct PCIe access, which requires PCIe tunneling instead of a USB-to-NVMe bridge mode. You'd have to find out what chipset the enclosure is using and check its technical data sheet to see.

          Even then it's not really worth the time. Are you going to be doing something that requires such fast speeds?

          • shake on 17/03/2025 - 11:24

            @Clear: Practical answer is probably not. And you're probably right that it's not worth worrying too much about HMB for my general use case.

            But having said that - I just figure that if I'm going to be spending on a USB4/TB4 enclosure anyway, then I may as well make sure I get an SSD that can maximise the transfer speeds.

            • Clear on 17/03/2025 - 11:32

              @shake: Realistically you're only to get around 3500-3800MB/s with an enclosure as they're going to be running at x4 speeds as opposed to what an M.2 slot on the MB can do.

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