Re: Pi5 M.2 HAT

Pancho <Pancho.Jones@proton.me> wrote:

I have three 2242 NVMe, they work fine, apart from some versions of
U-Boot boot loader (They actually worked in older versions, then stopped
working). A couple of those are 256GB from a couple of years ago, due to
the price low differential I would buy 512GB now.
 
I'm thinking of getting a M.2 NVMe adapter for my rPI5, I'll probably
get a Pimoroni one, because it take standard 2280 drives. Best to go
with the flow.

Agreed, if you don't need the small size then I'd go 2280 - plenty more to
choose from.

I think they've got them around the wrong way.  Their ODM Biwin's 2230 has
more read than write IOPS:
https://droix.co.uk/product/biwin-2230/
>

 
Yeahbut...
 
<https://www.scan.co.uk/products/512gb-wd-pc-sn740-m2-2230-pcie-40-x4-nvme-ssd-5000mb-s-read-4000mb-s-write-460k-800k-iops-tcg-pyrite>
 
650K  IOPS Max. Random Read 4K
800K  IOPS Max. Random Write 4K
 
 
But as I said, I don't really understand what IOPS means. The same
device quotes a faster Max Read than Max Write (presumably sustained
read/write).

4K random read test:
If the disc contains N blocks of size 4K
Repeat:
- roll a dice between 0 and N to give you D
- read 4K block number D from the disc
- throw away the data that came back
IOPS = how many times you can do that in a second

IOPS is a function of how well the flash and controller can manage an
unpredictable workload.  It's also a function of transfer speed to some
extent - you still need to move the data.  That 800K IOPS is 3.2GB/s or
26.2Gbps.  The Pi's single PCIe lane is only officially rated at Gen2 or
5Gbps, which puts a hard limit of 4K IOPS of 150K (and PCIe transfers have
additional overhead on top of that)

A more interesting graph in reviews is 'performance consistency': if you
give it a sustained write workload like this, after many minutes eventually
the speed falls as all the on-drive buffers fill up.  How well engineered it
is will show whether it can hold its performance, degrade gradually, hop
around erratically, or fall off a cliff.

Theo