Re: [tlug] auto-shutdown at certain temp, testing, and CF boot

[Edward Middleton <edward@example.com>]

Joe Larabell wrote:
> I'd add a third possibility:
>
>   
>> PROBLEM
>>
>> Making a system with a 3 disk RAID5 array bootable in the even of disk
>> failure.
>>
>> METHOD 1
>>
>> Create a partition on the start of all three drives and RAID1 mirror the
>> partitions[1]....
>>
>> METHOD 2
>>
>> Get an IDE flash card[3] place it on your first IDE port and put your
>> boot[4] partition and MBR on it...
>>     
>
> METHOD 3
>
> Take the simple way out and assume that once the disk fails, someone will
> have to visit the machine in person to replace the disk anyway so at that
> time they can also reboot the system from alternate media. Obviously, in
> the case of an unattended machine that simply *must* come back up on its
> own despite a failed disk, something else is needed. In my case, I really
> use raid as an additional later of protection for my data. It's a desktop
> machine so I can easily jam in a boot floppy in the case of a catastrophic
> failure. And because it's cool. YMMV.
>   

Your MTTF of either floppy or CD solution is going to be much shorter
then either a 3 disk  RAID1 or a IDE flash device, and the flash
solution also has the added advantage of allowing hardware right protect
your boot partition which will also protect you from some user errors or
software failures.

> Nonetheless, for the sake of completeness...
>
>   
>> 1. Partitions in a RAID1 mirrors can be directly booted from grub, lilo etc.
>>     
>
> I didn't know that. I'm skeptical about lilo, though. I thought grub's
> claim to fame was that it could read a real file system even without the
> kernel's help. The bootloader needs a lot more code than what can easily
> be stuffed into the MBR before it can access a raid array, no?
>   

It doesn't access the raid array, it accesses one partition read-only of
the raid array.  This can only be done with RAID1 because a mirrored
partition drive is almost identical to a normal partition.   All drives
will automatically have a complete copy of the boot partition because of
the RAID1 mirroring.

>> 2. You will also need to install a MBR on each drive.
>>     
>
> How does the BIOS know which MBR to load? In this case grub won't help
> because the MBR has to be loaded first. Are there BIOS'es now which will
> fail-over to another physical device if the primary boot device is dead?
>   

Any will boot. That is what the boot order is about, but whether fail
over works will depend on how the hard drive fails, and may require you
to manually changing the boot order.

> And, it the BIOS *is* that smart, would it not also be possible to keep
> a boot CD in the CDR drive while booting primarily off the non-redundant
> /boot partition? That keeps things simpler but still provides fairly good
> redundant boot capability in the case of the sudden death of the primary
> boot device. It's probably easier to convince the machine to boot from a
> CD than from a flash.
>   

The flash drive I pointed out emulates an IDE hard drive so it is
actually easier and more reliable then a CD boot.

>> 4. Boot partitions are mostly read-only so good flash memory should have
>> a greater MTBF
>>     
>
> That's a very good point. No moving parts. But it's still a single-point
> failure that *could* make the machine unbootable. Then again, the CPU or
> power supply is probably just as likely to go out, eh?
>   

If you are going to have a single point of failure you want the MTBF to
be very long.  This is not the case for a floppy, CD, or no-RAID
partition.  All have moving parts, floppy, and CD's are susceptible to
dust and general ware, even if you ignore the possibility of damaging
the media.  There is the added hassle of having to create boot partition
backup.

Edward