Linux won't boot from SAN after install

I had another issue with Dell's Poweredge 2950 and the combination on Oracle Enterprise Linux (OEL) 4 update 5 (4U5), qlogic 2460 single port HBAs and an EMC DMX SAN. I was trying to install a Linux server to boot from SAN... It seems be caused by Dell's BIOS and Linux device enumeration, about which I've reported earlier.
Dell uses DRAC to service its servers remotely. Using DRAC, you can mount virtual devices: a floppy and/or a CD (image). With these, you can install driver updates, install an OS or apply updates to a machine as if you're sitting directly with them. Very handy, but...
These virtual media get recognized as USB/SCSI devices, or so it seems, and this causes Linux to assign device names to them, just like ordinary (SCSI) disks. In my case, the Dell Virtual Floppy gets enumerated as /dev/sda, which bumps up the internal PERC controller and my QLogic HBA. This results in a boot drive letter change from /dev/sda to /dev/sdb or sdb to sdc. Therefore, your freshly installed system - which was fine until reboot - won't reboot. It's disks are gone and it can't find your root partition.

I did find a work-around though and hope this helps someone. First thing you'll need to do, is install everything inside LVM. I did and it seems LVM can handle partitions moving around by tagging its devices with a unique udev ID. If devices move to a different drive "letter", then LVM will be able to find it. The boot cycle will take a bit longer because it needs to scan for the IDs, but your system will boot.

Next, boot your machine, enter BIOS, set the "USB Drive Virtual Type" to 'harddisk' instead of (the intuitive) 'floppy'. This moves the virtual floppy in the harddisk boot sequence menu and you can use that menu to set the order of the disk controllers. I set my HBA as the first, PERC second and the virtual floppy last.

If you now (re)install Linux according to taste, using LVM to create a VolumeGroup and LogicalVolumes for your system partitions (lvroot, lvvar, lvtmp, etc.), you'll be fine. One more thing: after partitioning the disk(s), check the "advanced boot options" checkbox and set the boot order of your devices: put the LUN (here: sdb) before the local disk (here: sda).
Upon reboot the virtual floppy still gets inserted as /dev/sda, but the driver ignores it and LVM will find your root partition on /dev/sdc (vs /dev/sdb when you installed it).

More on Linux Enumeration of NICs.

Update: You can also solve this, if you're fine with the new order, by editing the grub menu and pointing grub to the proper new disk, i.e. hd(X,0) vs/ hd(0,0), where sda will be 0, sdb will be 1, etc. See man grub


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