Hi,
I wrote a private email to a friend. The content may be of interest to
some on the list, so here it is.
Do you know much about NVMe over PCIe for SSDs? I don't, only having
bought SATA SSDs in the past. The Pi 5 has a ×1 PCIe connector, meaning
one channel of two twisted pairs to allow a full-duplex connection.
A ×4 PCIe has four of them allowing data to be sent in parallel, 4 b at
a time. Raspberry Pi have recently released their PCIe HAT+ for
plugging in an M.2 NVMe SSD, though only in one of two short form
factors. https://www.raspberrypi.com/news/m-2-hat-on-sale-now-for-12/
That's handy. For $12 one can plug in large reliable storage and boot
from it. Crucial, really Micron, don't make M.2 SSDs that short, but
Samsung do so I'll probably plump for them as a second preference. But
it's only one drive. What would be really nice is two in a RAID 1
mirror.
The two peers sat at either end of the PCIe can negotiate the ×푛
channels downwards. This is how a ×4 can plug into a ×16. Some
controllers can also support ‘bifurcation’, splitting a ×16, say, into
a ×8 and two ×4 so a passive adapter card can plug into the ×16 and
provide three connectors. This also needs BIOS or OS support.
(Bifurcation seems the wrong word but perhaps it's split by a multiple
of two each time.)
(Sorry if you know all this. By putting it in writing, I'm having to
think it through for myself.)
Anyway, the Pi 5's ×1 can't be split. So a passive adapter, one which
is effectively just wiring, wouldn't work. My slight understanding is
an active adapter, meaning it has a PCIe controller, is required to
bring forth a ×16 or whatever by intelligently multiplex. Obviously,
it's all squeezed down a ×1 in the end, but if performance was key then
the Pi 5 wouldn't be used. It's just a compact way to get considerable
RAID 1 storage to hand.
Ah, I see a reference to a ‘PCI switch’ rather than controller to
multiplex. That's led me to
https://pineboards.io/products/hatdrive-dual-2xnvme-2230-2242-gen-2-for-raspberry-pi-5
which uses an ASMedia device. So such things do exist. €40 seems
a little high, but they're low volume.
NVMe has other standardised transports, e.g. TCP/IP with TLS. This
suggests it's just a ‘simple’ matter of software to have a RAID 1 with
one M.2 drive dangling off the local ×1 PCIe and the other over
Ethernet. Ethernet can stretch from LAN to WAN: a second local Pi 5 on
the back of the TV running LibreElec, or an off-site non-Pi.
The Linux kernel has NVMe support since 5.0; package nvme-cli drives it.
There's also the BSD-licensed SPDK which is a user-mode C implementation
of NMVe along with a network stack; to avoid the kernel overhead.
https://spdk.io/doc/about.html. Which makes me wonder what the minimal
hardware is to deploy either and drive local NVMe storage for network
access. Not corporate-level data-centre SAN but home, hobbyist,
small-office level. Easiest would be some SoC, ARM or RISC-V, running
Linux. SPDK on a non-Linux SoC with PCIe would seem possible.
https://spdk.io/doc/porting.html
--
Cheers, Ralph.
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