Thanks, that’s very kind. I’ve added some more detail on other replies and I think I’ll do a full blog post in the next couple of days.

There are definitely parallels with the Compute Blade project but there are a few differences. My blades are a bit simpler, they don’t have the TPM that the Compute Blade does, as I didn’t have any real need for it. The CB also has a more dense number of blades in the 19" width. This was another design decision on my part, I quite liked the short depth case making the unit small and I wanted to make sure there was plenty of airflow for cooling (tbh I didn’t need as much as I used!)

My unit is more focused on being like a traditional server unit, as that’s what my use case was. Centralised power, centralised management and provisioning etc. You’re correct, the Compute Blade uses PoE, and I did it through the backplane. My preference was for central management rather than per-blade, so that meant a backplane and it all flowed from there. It allows you to feed the USB and serial console into the management server which is great for provisioning and debugging. The displays are also born out of my days as a network infrastructure guy, where being able to see the server’s name and IP address on the physical unit would have been a godsend when doing maintenance! So I guess the design differences between this and the Compute Blade are about my focus on more of a server use rather than general compute module.

I’d say it’s probably a bit cheaper using a backplane than PoE. The PoE adds a bit to the cost of each blade which would soon multiply up, plus the additional cost of a PoE switch vs non-PoE. I’m using an off-the-shelf ATX PSU and these are made in such huge quantities that the price per watt is difficult to beat.

Yeah, this isn’t useful for many things, but as others have mentioned there are situations where it is. My original use case, the thing which prompted me to build this (other than just the fun of seeing if I could do it!) was to replace a whole load of low complexity VMs. I’m a freelance programmer and I do a bunch of hosting for both myself and some clients out of my home office. I’ve got a small rack setup in my attic with UPS, and have redundant fibre connections. It’s obvs nowhere near datacentre quality but it works well for my purposes.

I’d previously been using VMs running on some second hand enterprise x64 kit that I bought. Whilst this works great, the electricity bill is rather higher than I’d like! When I analysed what all the VMs are doing I realised that it’d be perfectly possible to do this on a Pi. In the dim and distant past I was a network infrastructure guy, so I started looking into “proper” server Pi solutions and before I knew it I was down this rabbit hole!

It works really well for low power server applications. It’s not in the same league as the big iron ARM mega-core servers (or indeed Xeon servers) for performance, but then it’s nowhere near that league for price either. I haven’t figured out an exact price if I was to sell it commercially, but it’d likely be in the $800 US price range without CMs. If you were to max that with 4GB PIs that’d end up around $1500, which’d give you 40 cores of pretty decent performance and 80GB of RAM. The Gigabyte and Altera servers I’ve seen are awesome and way more powerful than this but are several times more expensive.

Probably a k3s cluster. The bigger constraint will be memory.

Thanks, some more info on other replies and I’ll do a proper blog write up in the next few days.

The Pi shortage was definitely a nightmare. I wouldn’t have bothered with this even at the start of the year as you just couldn’t find the kit. It does seem to have eased now, though. Digi-Key and Farnell have had good stock levels of CM4s for the last couple of months and the CM5 can’t be far away now. I’ve found performance of CM4s with NVMe SSD’s are pretty good, certainly enough for my use cases. Blades like this aren’t much use for storage servers though, not really enough storage options. I wouldn’t rule out doing a purpose built storage server at some point though, my home network has a couple of big NAS boxes with 20-odd SSDs and I’d love to replace those!

Yeah, at the very least I’ll hand-build a few units with the spares I’ve got here and make those available. If there’s enough demand I’ll potentially do a full production run. I’ll open source the designs too so folks can have a proper poke about in it :-)

Thank you! Yes, you’re correct on your guesses. There’s blade to backplane/management server comms, but no direct blade to blade comms. As I’ve mentioned on a couple of other replies, it’s definitely possible to do a version of this where the Ethernet comes from the blade to the backplane over the PCI-e connector and into a switch on the backplane, so that you’d have all the switching done on-board and a single uplink port. It’s a much more complicated project to do though so not something I’ve tackled yet.

The blade uses PCI-e card edge connectors as they’re cheap, and I route UART0 (GPIO 14/15) and the USB from the compute onto this. There’s a USB switch IC on the blade which can route the CM’s USB output to either the host port on the front of the blade or through the backplane. The UARTs and USB are connected through switches on the backplane into the management module. The blades also have RP2040s on them which are connected to various pins on the compute modules, and the management module can talk to these using I2C. It’s able to use this for doing stuff like restarting the CM into provisioning mode, and for reporting status information. The RP2040 is connected via I2C to both the compute module on the blade and the backplane’s management module, so can be used for exchanging status information from within Linux on the blade with the management module. That’s how I get out status, temperature etc info. There’s no reason this couldn’t be used for other stuff too, and in theory could be used to exchange inter-blade data at I2C data rates.

The connector also passes out the RP2040’s UART and SWD as I use this to flash the firmware into the RP2040. I haven’t switched this into the backplane but in theory it could be too.

Yeah I know about that one, I looked at it when I first started thinking about using Pi’s to do the server stuff I wanted, but I couldn’t actually buy one then. So I built my own :-) As I mentioned on another post, there’s a few differences around my focus on using this as a simple server system.

First thing that came to mind as well

Thank you, much appreciated :-)

Thank you! Yes, I’d really like to do this in a future version, I’ve mentioned a bit more detail on some other replies. It’ll be quite an expensive thing to develop which is why I went with this for now. It’d certainly make for a much tidier setup.

The feedback’s been good so I’ll hopefully get this available to buy soon. I think somewhere in the $800ish price range without the CMs, which hopefully is near to home use price range.

If you were to lose the front panel sockets and do the switching on board then it’s possible to pack quite a few into a 19" width. There’s also space to fit 2 compute modules per blade, so in theory it could get quite dense. Cooling would definitely be the issue. I suspect with powerful enough case fans it should be workable.