RAID, RAID metadata and /dev/dmXX

[Tapewolf]

About a year ago I set up a small RAID mirror using the onboard nVidia controller.  It seems to work equally well via dmraid in linux and whatever Windows 7 does.  Recently I have been thinking about an upcoming move to Bulldozer when that comes out and it is likely that the replacement motherboard will not have an nVidia chipset.

So, I imaged the mirror to a spare disk and moved the array over to a Silicon Image PCI controller and set up a new mirror.  However, it seems to be quite happily running with the old nVidia metadata, in both linux and windows.  This concerns me a little.  Should I be wiping down the entire array and recreating it in SIL format, or is this safe?

On a related note, I notice that all the volumes on the mirror are mounted as /dev/mapper/nvidia_cgixxxxxN except for the Windows partition, which has mounted as /dev/dm11 instead.  What's with that...?

[ShadesFox]

I'm pretty sure that, when it comes to desktop PC RAID controllers, they don't actually have their own metadata.  In fact, I'm fairly convinced that they do nothing other then being informational to the BIOS that a RAID setup is expected.  So I'm pretty sure that what your doing is safe, since you are in SIL or nV format, but rather in Windows/dmraid format.

[Tapewolf]

I'm pretty sure that, when it comes to desktop PC RAID controllers, they don't actually have their own metadata.  In fact, I'm fairly convinced that they do nothing other then being informational to the BIOS that a RAID setup is expected.  So I'm pretty sure that what your doing is safe, since you are in SIL or nV format, but rather in Windows/dmraid format.

I did wonder, since I was expecting the array to be completely garbled after a controller switch.  That said, when it first came online, dmraid reported something like "/dev/sdd: found nVidia and SIL formats!  Using nVidia".
After a bit of messing around I used a command to try and erase/rebuild sdd's metadata in SIL format.  That worked insofar as it no longer said that and dmraid still thinks the drives are mirroring, though it does make me worry that the two drives might be in different formats or something equally weird.
FWIW it still reports both as 'nvidia: /dev/mapper/nvidia_cgi......."

When I am able to get hold of another power adapter to add a third unit to the array, I should be able to try things like pulling one of the disks for a while and seeing whether it rebuilds.

I would be interested to know about /dev/dm though.

[RobbieThe1st]

I can say a couple of things about this, having once done the exact same thing:
1. Nvidia's fakeraid is really lax about it's metadata(at least for raid-1) - you can literally yank a drive, plug it into a USB adapter, and it'll act just like a single drive. And most of the time(provided you didn't write anything), it won't do a rebuild on reconnect
2. I think the only part of it that it checks is some weird superblock on the disk; Of the three current disks I have in a software raid, one still thinks it's a Nvidia raid volume, despite having all data on the drive wiped through formatting the final raid-5(not zero'd, though), and a new partition table written.
3. If you're just going to use it for Linux, go for a soft md raid. You'll get better performance(though still around 1 drive's speed for raid-1; raid-5 gives *much* better performance) For windows, you may be stuck with the hard raid, or you may be able to use the disk manager and go for a windows softraid also.

[Tapewolf]

3. If you're just going to use it for Linux, go for a soft md raid. You'll get better performance(though still around 1 drive's speed for raid-1; raid-5 gives *much* better performance) For windows, you may be stuck with the hard raid, or you may be able to use the disk manager and go for a windows softraid also.

I did in the end, yes.  Moved windows onto a disk all of its own, though it self-destructed in the process.