Can I upgrade Ram SG125 ?

There is RAM specialist vendor who lists modules and specifications datasheets for each per SG model and Revision phs-memory.com / speicher.de Sorry no deeplinks, see BAlfons' remark on ths

They specify as follows, so we probably do not need to open our boxes. However if you do, it would be interesting to know if they have two modules with 2GB or one module with 4GB.

Rev.2 has the following specs:

Device - Specifications

Computer memory - info

whereas the

Rev.3 has the following specs:

Device - Specifications

Computer memory - info

So order 2 x 8GB off ebay and stick them in.

The SG125 is an old beast and the DDR3 8GB sticks are cheap to be had these days that you can't go wrong. Providing you buy the right spec. 

Yeah RAM is dirt cheap these days for these smaller modules.

My SG is Rev3 and still in use though and I guess just 2GB more would be enough as the 75% RAM usage warnings are annoying. Another 4GB would do the trick for old UTM firmware for sure and reduce swapping. Of course different use case is if somebody wants to re-use it as a server and virtualize some machines.

I wonder if it could be upgraded to ECC as the underlying barebone seems to support ECC, based on the information the manufacturer is publishing: DNA1160/DNB1160 Again removing hyperlink: nexcom.com/Products/network-and-communication-solutions/cyber-security-solutions/desktop-x86-based-appliance/desktop-x86-based-appliance-dna-1160/Specifications

And here a small sneak peak into the SG125w Rev3: community.sophos.com/.../mini-pcie-slot-use-on-xg-125-rev-3 It is actually high res if you open the picture in a seperate window.

Keep in mind, that Rev3 (with HDMI port) and Rev1/2 (with VGA port) as mentioned above don't share the same spec.
The earlier revisions are based on the NexGen DNA-1150 with the infamous C2358 and it's AVR54 LPC clock output bug.

Firewall's are pretty premative when it comes to speciality stuff. I used to run a pfSense firewall at home on a M400 and prior on the other older WatchGuard stuff, I even got another off brand built in Canada by some company and its hardware was similar. In the end for my home (not my office that uses Sophos) I ended up getting a Unifi UDM-Pro. Since Unifi is made of ex-Apple networking folks you can probably understand their hardware is all speciality stuff! LOL

My point is you are best to go with the recommend and not try to go off the rails too much, adding ECC would be an off the rails thing.

I found this older document for technical specs:

2015 sophos-xg-sg-all-models-tech-specs.pdf

I can tell from my own experience after upgrading an older model SG105 rev.1: this device has a single SO-DIMM in DDR3 technology.

So I ordered a Kingston 8GB DDR3-1600MHZ (KCP3L16SD8/8) to replace the original 2 GB, worked perfectly.
This is now our "Lab" firewall to play around with.

I will do some testing with the Rev. 3 and order Crucial RAM 8GB DDR4 3200MHz CL22 CT8G4DFRA32A 

It is a 3200Mhz Module, but should be backward compatible with 2400Mhz. I wish I had some ECC modules lying around to test, but I am not buying them on chance.

Can anyone verify that on the SG125 Rev 3 it is a Intel Atom C3508 or can tell me if lshw or hwinfo are installed on the UTM for command line?

If it is a Intel Atom C3508 it would even be only DDR 1866Mhz

Both utilities exist.