RAID and distros

raungst · n00b Joined: 20 Mar 2006 Posts: 37 Location: Denver, CO

My gentoo server currently runs a RAID 1 array. I have a separate hard drive that runs the system, and the array is simply storage for the server. I've contemplated changing to a different linux distro, but what impact would this have on the RAID? For some reason, I thought the initialization was tied to the distribution..ie, if I installed a new distro on the system hard drive, I would have to reinitialize the array for it to work correctly. Is this the case?

Or, if I setup the device nodes, fstab, mdadm, etc correctly, will a new distro just see the array like nothing happened?

I apologize if this is a stupid question.

Thanks!

eccerr0r · Posted: Tue Jul 24, 2007 12:50 am Post subject:

I'd venture to guess that most distributions will find the RAID and give up on what to do with it... It may install fine but then it will get the boot sector wrong.

otherwise it should be the same underlying code that handles the software RAID.
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raungst · n00b Joined: 20 Mar 2006 Posts: 37 Location: Denver, CO

I don't understand why the boot sector would be involved. My system's hard drive is separate from the RAID. The two hard drives connected to the RAID 1 just have one big ext3 partition.

eccerr0r · Posted: Tue Jul 24, 2007 5:26 am Post subject:

Oh, if it's totally separate then likely you can just ignore the disks and install to the other disk, re-enabling it later. All linux if they use the same kernel and same version software RAID, the disksets are interchangeable.

I boot off of my RAID5 disk set, thought it was more common to actually boot off of your array. I'm fairly certain most linux distributions will get my RAID setup wrong.
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HeissFuss · Guru Joined: 11 Jan 2005 Posts: 414

You shouldn't have a problem with the raid array as long as you're careful to install on the correct drive. If you have it set up with md raid and partitions of type autodetect, the new distro will likely just detect and bring up the raid for you. A good way to test this is to boot into the new distro's livecd (since I assume you're probably going to Ubuntu or another distro with a livecd) and see if your array gets activated (simple test: cat /proc/mdstat.) If you have a non autodetect raid, you may need to save and import your /etc/raidtab or /etc/mdadm.conf or whatever config happens to have the array layout stored in it.

Jkay · Posted: Tue Jul 24, 2007 6:15 am Post subject:

Curious .. I have attempted several times over the past ten years to get random Linux distros to boot from my raid5 array, but all install and then fail to boot on the first "live boot" ... what's the cause and the solution? I used RedHat for years and then moved to Gentoo and both have screwed up in this manner, so I always boot from SATA/IDE and then mount the scsi raid5.

Rob1n · Posted: Tue Jul 24, 2007 8:26 am Post subject:

AFAIK, you can't boot directly from a software RAID-5 array. Neither LILO nor GRUB understand RAID arrays, so unless the /boot directory is on a single disk then it can't be accessed. RAID-1 can be booted from as the boot manager can treat them as separate devices (all the data is replicated so either drive has the full /boot directory) - you should install grub on both disks though for redundancy.

eccerr0r · Posted: Tue Jul 24, 2007 5:48 pm Post subject:

Right, my setup is weird. You can't boot directly off a raid5 array. However the whole disk doesn't need to be RAID5.

I have a 4-disk RAID5, with a 4-way RAID1 boot partition on that array. This is where I install GRUB on. So basically if any one of the disks die, I still have a 3-way RAID1 to boot off of, and the 3 remaining RAID5 partitions. Just hope that the auto-failover of BIOS will try the other disks.

I'm not certain if any distro would auto-detect this setup or not...
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Rob1n · Posted: Tue Jul 24, 2007 6:03 pm Post subject:

obrut<- · Posted: Wed Jul 25, 2007 11:45 am Post subject:

i built a 3 disk set with raid1 and several raid5s using raidtools (i guess more than 2 years ago) on another gentoo box. a few month ago i put it into another pc also running gentoo. at first the array was not autodetected and the raidtab was on one of these arrays :twisted:

i then compiled the necessary stuff as part of the kernel and not as modules and the arrays were correctly detected the next time i rebooted. i don't know if mdadm or raidttools (whatever tool you used for building the array(s)) need to be installed for just mounting the arrays, but they won't hurt i guess.
because the arrays' metadata, the raid superblock, (which partitons belong to the array, their order, raid level, etc) are stored on the partitions itself. when theses metadata are written to all partitions (i think i had to tell the raidtools to do so) the kernel should not have any problem to automatically detect and reassemble the array.

eccerr0r · Posted: Wed Jul 25, 2007 8:16 pm Post subject:

I guess it's not the fact whether or not it can autodetect, since it's the same Linux. However whether or not a specific distribution is smart enough to install the bootloader on the RAID1 and not bother trying to either install on /dev/hda or the RAID5, remains unclear...
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NeddySeagoon · Posted: Wed Jul 25, 2007 8:32 pm Post subject:

eccerr0r,

What sort of raid1 ? Kernel raid or BIOS software raid or even real hardware raid ?

You can boot from BIOS software raid (all raid levels) real hardware raid if the BIOS sees the controller (all raid levels) as these sorts of raid hide the data layout on the drives from grub. As you are using dmraid or a kernel module for your raid card, its distro agnostic., If it works for one it will work for them all.

Kernel raid is a bit harder. Its the same kernel modules for everyone but theres a difference. The raid does not exist until the kernel is running and grub must work before that time. Thus /boot must be raid1 or unraided, as grub cannot read kernel raid at all.
root and everything else can be any level you like because the kernel forms the raid sets before it tries to mount root.
These rules apply to all distros too.

In short, if you follow the rules for your sort of raid, it doesn't matter what distro you use.
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