OpenBSD Software RAID Administration: Caveats

• Introduction
• Overview
• Installing OpenBSD
• Dealing with a Failed Component
• Upgrading OpenBSD
• Caveats
• References

Caveats

Some hints, tips, and gotchas:

• Kernel modifications: Keep in mind that the kernel on the RAID set is NOT the kernel that gets booted. The boot filesystems (wd0a and wd1a) contain the kernels. If it's necessary to make changes to the kernel, mount the boot filesystems and change the kernel(s) on them (making sure that you make identical changes to both filesystems so they stay in sync).
• /etc/fstab: If you add /dev/wd0a or /dev/wd1a to /etc/fstab, ensure that the sixth field is set to "0" (zero) so that fsck(8) doesn't try to check the drive on boot.

  /dev/wd0a / ffs rw 1 0

  If either wd0 or wd1 fail and the system is rebooted, fsck(8) will try to check the failed drive. If it's unable to find the drive, the system will stop booting and drop into single user mode. For further information, see the fstab(5) manpage.
• Parity Rebuild on Boot: If the parity on the RAID set needs to be rebuilt during system boot, the rebuild will start just before the filesystems are mounted. This process halts the boot, and by default, doesn't give any status output. This can be changed by adding "v" to the raidctl arguments in /etc/rc. e.g.: raidctl -vP all
• BIOS Boot Devices: It's important that the BIOS try to boot from the second hard drive in the event that the first drive has failed. If the BIOS is set to only boot from the first drive and that drive fails, the system will not boot until the failed drive is replaced (and the RAID set repaired) or someone intervenes on the console during boot.
• RAID Set Size: Unless the system being built really needs gigs and gigs of storage space, keep the size of the RAID set small! Even if this means under-utilizing the drive, the smaller RAID set will take less time to build parity on and less time to rebuild after replacing a failed component.
• Converting to/from RAIDframe: Be aware that once a system is up and running it's a non-trivial task to convert to or from using RAIDframe. This task is better done at system install time.
• Alternate Root Device: It is possible to boot the RAID-enabled kernel without mounting raid0a as the root device, even if raid0a has auto-configure enabled. When the system is booting, at the boot> prompt, type boot -a. As per the boot(8) manpage:

  -a   Causes the kernel to ask for the root device to use.

  When prompted, enter wd0a or wd1a.