Re: Linux hard drive/device nodes for a Postgres RAID

On Thu, 2006-11-16 at 14:56, Martijn van Oosterhout wrote:
> On Thu, Nov 16, 2006 at 12:40:41PM -0800, Glen Parker wrote:
> > But now, pull the drive from port 2 and boot the system. You will now
> > have SDA,SDB,SDC. The kernel will now fail BOTH of the last two drives
> > from the RAID array. The one that was SDC is gone, and obviously fails.
> > The one that was SDD is now SDC, so its ID doesn't match what the
> > kernel thought it should be, so it fails it too. If you kill the FIRST
> > drive in the array, I believe the entire array becomes inoperable
> > because of the resulting shift and ID mismatch.
>
> Is that really so? AIUI the position of the disk in the array is stored
> on the disk itself, so it should be able to handle disks moving around
> no problem, have you tried it?

Just FYI, I've tried this before. yes, linux software RAID, knowing
that the linux scsi numbering system is non-deterministic, is designed
to handle this.

In fact, you can build a RAID5 or RAID0 array of as many disks as you
like, shut down the machine, change every single drive ID, and the
machine will still find the RAID arrays.

Last I tested this was on something like RH 7.2 by the way. Times may
have changed, but I can't imagine someone being stupid enough to break
the RAID array handling that worked so well back then.

>
> > So the question is, is there some way to "pin" a drive to a device
> > mapping? In other words, is there a way to force the drive on port 0 to
> > always be SDA, and the drive on port 2 to always be SDC, even if the
> > drive on port 1 fails or is pulled?
>
> I thought you could do this with options on the command-line, or using
> udev. But I don't think it's actually necessary.

You can, it's generally not.