On 29/01/18 09:36, Mark Rogers wrote:
Booting from an array created by mdadm is certainly possible and should avoid the SPOF issue.
Yes & no. My current machine boots fine from MDADM. The new one (unless I'm missing something) doesn't.
Differences, old one 32 bit, disks 2TB, new one 64 bit, disks >2.2TB.
I think the disk size is the problem. The old machine has a "DOS" style partition table. Grub2 installs its boot loader somewhere after the MBR record, and consequently can boot from this.
The new machine, the disks are too big for that style of partition table. I could perhaps ignore the extra space, but I did buy bigger disks and wanted to use them. I have consequently to use GPT partitions.
If I boot the machine in BIOS mode, a BIOS boot partition is required in order to boot. It appears to be impossible to boot from this if it is (software) raided. E.g. https://serverfault.com/questions/749274/is-it-possible-and-wise-to-put-the-...
If the machine boots in UEFI mode, a partition called /boot/efi is required. It appears that this partition cannot be (software) raided. E.g. https://askubuntu.com/questions/66637/can-the-efi-system-partition-be-raided
BUT, it seems someone found out how by https://askubuntu.com/questions/660023/how-to-install-ubuntu-14-04-16-04-64-... BUT BUT, the UEFI partition is NOT raided, just cloned using DD.
It's been a while since I've done it; where I'm using software RAID it tends to be just for storage and I have a separate boot drive simply because there's nothing of substance on it, so when it fails I use it as an excuse to rebuild. Desktop machines where the O/S partition contains stuff I don't want to lose are backed up but not RAIDed so I've not needed to boot from software RAID for a couple of years.
Fair enough, but I wanted the whole thing RAIDed. I don't particularly want a separate boot disk, as we're back to a SPOF again.
For Ubuntu* I found the following which looks like it covers configuring RAID at installation time: https://help.ubuntu.com/community/Installation/SoftwareRAID
Thanks for looking. I have used that guide in the past. In fact I may have used it when I set up my previous machine. Unfortunately it doesn't help with this situation.
If you have a desktop kicking around with something like VirtualBox installed you can of-course play with this in a virtual machine before messing around with real hardware. I recall it being a bit tricky only because it was outside my knowledge but actually fairly straightforward otherwise.
I don't have a machine around with enough guts to run VirtualBox, but I do have the new machine, which is not yet set up to do anything, so it's not a problem using this to try things.
*Other distros are available, of-course.
Of course! Thanks for replying Mark,
SO, I've managed to get a system set up with a UEFI partition that's cloned using DD, / and /home RAID1'ed with btrfs RAID, and I'm going to set up a SWAP mdadm partition.
But I could go to a system set up with a UEFI partition that's cloned using DD, / and /home and SWAP RAID1'ed.
Pros and Cons MDADM, mature technology. Will boot into degraded mode. I've used it before & understand it. BTRFS, advantaged for COW (Copy on Write), resistant to BIT-ROW. but I've not used it before and don't understand it. With disk failure, it stops, apparently at the initramfs/busybox prompt waiting for you to fix it before continuing.
SO, do I persevere with btrfs or do I go back to mdadm?
Any more comments??
Steve