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Re: Woody root on LVM on RAID


On Thu, 31 Jan 2002, Calyth wrote:

> Although I don't have the experience, I'd advise against using a soft-RAID root
> partition because if you don't have the right kernel for recovery.... you could
> be in pretty big trouble.

I've been running potato with the root fs on RAID-5 on several servers.
You could be in trouble if you had no rescue floppy.  But I found it was a
cinch to replace the standard kernel image on the potato rescue floppy
with an image of the RAID kernel I'm using (2.2.19 with the RAID-2 patch).

I checked that I could boot from it, mount all my filesystems and
generally do anything you could do with the standard potato rescue floppy,
short of installing from scratch...

> You're better off using a separate drive as a root partition for LVM or
> soft-RAID.

...Hence, I don't agree with your statement.  RAID is for providing
resilience, not data protection; tape backups (which, on a server, you
should be doing anyway) protect data.  If you're running the root fs on a
single device and it fails, you'll have to restore the system from backup
or rebuild from scratch.  After that, restoring the data from backup is a
cinch, so you might as well not run RAID at all.

Bitter experience speaks, not I.  A server we had here, set up with root
fs on one disk and data on RAID, failed when the disk with the root fs
went kaput.  Not only did my predecessor as admin (I was then his
assistant) go through ground-up rebuild hell, with all the users on his
case, he also had a lot of explaining to do to some very un-impressed
bosses, who were asking why they'd paid money for an array of disks when a
single disk failure could take the server down for a complete OS rebuild,
and why we were using RAID to protect data which was on tape backup
anyway, instead of providing resilience.

I learnt the lesson well.  My servers are now redundant right down to the
MBR's; you can use the SCSI BIOS to switch boot disks, and boot from any
of the five disks in the array.  The machine continues to run, and is able
to reboot, after failure of any one disk.  I have tested this by simply
withdrawing a disk (hot-swap SCSI).  You may have to use the BIOS to
switch boot disks if the failed disk happens to be the one the machine was
configured to boot from, but that's all.  All this with potato and
kernel 2.2.19.

With woody and kernel 2.4.17, I have fond hopes of combining the
redundancy achieved with Linux Software RAID with the flexibility of LVM
to build a real enterprise-level server.

Best regards,

George Karaolides

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