[mythtv-users] Mirroring via LVM or RAID?

[chris at cpr.homelinux.net]

On Sat, Jul 08, 2006 at 12:14:20AM -0500, Sage wrote:
> Regardless of whether you choose to use LVM, software RAID is probably
> what you want for redundancy.  Set your 400 GB disks up with two
> partitions each; one software RAID partition each and one swap
> partition each (It's a bad idea to put your swap partition on top of a
> software RAID array -- I've seen stability problems from it firsthand,
> and you don't need swap redundancy; just having two separate swap
> partitions is enough).

If you don't have swap redundancy then a drive failure is likely to 
cause your kernel to panic or lock up.  If you are putting in 
mirroring for data retention then that may not be too bad, but if 
you plan to use mirroring to allow yourself enough up-time to 
perform a controlled shutdown then you're better off putting the 
swap inside the mirrored space so that the kernel can survive the 
failure.  There is an impact on swap performance, of course.

> 2. Even if you have your /boot partition on a software RAID 1, the
> bootloader is only installed on one disk.  Which means that if your
> first disk dies, even if the other disk is still alive, you can't boot
> into it.  You can fix this by manually installing grub onto the second
> disk, but you have to remember to do so every time you update grub
> (which probably won't be often).

If you use lilo instead of grub then it will install the boot data 
on both drives of a raid mirror so you never have to remember to do 
a second update.  Since both drives are bootable, the loss of hda 
doesn't prevent you from booting the machine (although you will 
need to swap cables if your BIOS doesn't support booting from the 
second interface).

> As for hardware RAID, I would actually recommend against it.

I agree for all the same reasons plus one you missed: hardware raid 
is very hardware-specific.  If your raid card dies and the new one 
is a different make/model then you lose all your data.  Sometimes 
even a firmware revision is enough to make the new card 
incompatible with the old array.  With software raid you can make 
all kinds of hardware changes and be reasonably certain that the 
system will still boot because the operating system accesses the 
physical drives independently.