Fake-Raid or not Fake-Raid, that is the question…

Last week I was building a cheap and cheerful file server using a software iSCSI target, a workstation and some off the shelf SATA disks. I have done it before with great results, but sadly my preferred iSCSI target vendor has recently stopped selling the low cost edition of their product and the new “entry level” is almost three times the cost – forcing me to re-examine the competition.

I will leave the comparison of other target vendors for another blog – this is more of a flashing red “Danger Will Robinson!” warning to others using workstation RAID configurations for such tasks so they don’t get caught out as well.

The workstation used was a Dell Precision 390, their top end enterprise level machine from a couple of years ago. It comes with an onboard Intel raid card capable of RAID0/RAID1 and RAID5 and I have used it with success on Windows platforms in the past. However while I was exploring other iSCSI vendors I tried to install a Linux based platform and discovered it was ignoring the RAID configuration and able to examine the individual disks.

There were five disks in total:

• RAID1: 2x 250GB (OS)
• RAID5: 3x 1TB (DATA)

Rather than seeing two disks (OS and DATA) it saw all five disks disregarding the RAID. A little bit of digging demonstrated why and left me quite red faced. Apparently taking their lead from the type of marketing teams responsible for claiming their graphics cards have 256MB RAM when actually they have 128MB (the rest is optional shared memory) – the vendors are trying to claim their solutions are more than they actually are.

In fact they are not actually RAID cards as we know them at all, they might have a setup/configuration menu in the BIOS, but all they are actually doing is passing that configuration to the OS (ie – Windows) which has a driver aware of the configuration and handles the RAID – this means that it is software RAID and not “real” raid. The reason Linux is not seeing the raid configuration is its driver does not support it.

The fact this is happening seems immediately obvious armed with that information. A “real” RAID card will handle the processing on a dedicated processor and have a dedicated RAM cache for write-back operations to increase performance. I’m looking at the motherboard right now and can see no dedicated RAM cache or processor – so all operations are actually using my precious system resources just the same as if I was to configure software raid from within Windows.

Hopefully reading this will prevent others from going through the same thing. Raid cards are expensive for a reason. I will be posting performance statistics over the next couple of days if anyone is interested.

Go forth and buy “actual” RAID cards!