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I have an xenserver and I wonder to know if it's possible to boot it from a RAID5 software partition.

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Not an answer to your question (David gave one below), but, please - consider adding two more hdd's and create a separate raid1 array for your OS. You really do not want virtual machines clogging up your service console I/O. – pauska Nov 14 '09 at 12:22

Nope, sorry. But there are lots of alternatives.

You could create a small partition at the start of the drive(s) and store the boot loader there.

You could get yourself a cheap flash drive, and mount it in the case and put the boot loader on the flash drive.

You could even deliver the kernel via the network. PXE booting is pretty easy to setup.

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So Linux generally doesn't authorize the boot from RAID5? It's possible only from RAID1 devices? – Proxium Nov 14 '09 at 1:49
The problem isn't really with Linux. It is that the BIOS of your computer and the bootloader cannot read the software RAID. It can boot from a RAID1 because it doesn't need to know anything about the RAID. – Zoredache Nov 14 '09 at 1:56
@Proxium: Linux has nothing to do with it. The common boot loaders, LILO and GRUB Legacy, have no knowledge of software RAID. RAID1 only works because GRUB can ignore the RAIDyness of it and just treat the boot drive like a regular drive. – womble Nov 14 '09 at 5:01

No you can't. This is because when the system boots (as in, takes the hand-off from POST) it doesn't know that the sequential blocks of the file are spread across disks. It needs to load the kernel first before it can handle a file system spread out like that. As advised by Zoredache, you can probably create yourself a little /boot partition for your kernel on the first disk, that isn't RAID-5'd and then RAID the rest. This will require careful consideration of your RAID slice layout.

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For completeness, some software RAIDs can boot a stripeset - Solaris ZFS and Mac OS X come to mind. Both use a double-bootstrap technique where enough information to bootstrap the stripe set is on all the disks - like tiny boot volumes.

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