[lug] RAID redundancy with boot sectors?

Dan Ferris dan at usrsbin.com
Sun Nov 26 07:25:06 MST 2006

You are right except for one thing that we did a lot of at CU, which was 
to use Linux software RAID 0.  We used to make a lot of software RAID 0 
arrays to increase performance.  Since you can only mirror /boot, that's 
what we did.  I do remember it saved me once, when the boot sector on 
one drive got hosed and I booted from the second drive and restored it.

I've messed with those highpoint cards before.  They are the worlds 
worst pieces of junk.


Nate Duehr wrote:
> Dan Ferris wrote:
>> With Redhat based distros, you can only make the /boot partition on a 
>> RAID 1 if you decide to use RAID.  You can't make it anything except 
>> for RAID 1 if you want to use RAID.  When Redhat goes to format the 
>> partitions, it will install grub on both partitions that are going to 
>> be the /boot RAID 1 mirror.
>> I've had it screw up as well.  When that happens, you just use dd 
>> like this:
>> dd if=/dev/sda of=/dev/sdb bs=512 count=1
>> If the boot drive dies, you just go into the BIOS and tell the PC to 
>> boot off of the second drive and you should be good to go.  It works 
>> especially great with SATA drives.  I've done this procedure twice.  
>> Once where Fedora didn't install grub on both drives and I had to 
>> manually install it , the second time it worked flawlessly.  Since 
>> your /boot is mirrored, the kernel images will all be where you 
>> expect, you just have to make sure to install grub.
>> This should work even if you are using RAID 5, since you can only 
>> mirror /boot.
>> Incidently, this is a good reason to always have a seperate /boot 
>> partition.
>> Another thing is that if you have a RAID 1 mirror and a drive dies, 
>> you can swap the bad one out, dd copy the partition table from the 
>> good drive to the new drive and you should be able to rebuild the 
>> mirror very rapidly.
> Hi Dan,
> I messed around with this for a while (software RAID 1 mirroring on 
> Linux) and ultimately came to the conclusion that it was utterly 
> useless.  But here's my criteria...
> In the case of IDE or SATA, a disk failure generally has a tendency to 
> lock the machine up anyway, if you're using bog-standard 
> (non-hardware-RAID) interfaces.  If not locked up, so degraded that 
> the box isn't working right anyway.
> "Real" RAID's whole purpose in life was to add uptime.  A disk fails, 
> it's removed and replaced seamlessly.  There's almost no way to do 
> this with the "Simple" software RAID.  Either you're swapping cables, 
> or pulling a disk out and rebooting (possibly with an fsck pass if the 
> box went down in a particularly nasty way), etc.  So you don't gain 
> the "uptime" factor.
> An equally simple setup that has almost the same amount of downtime, 
> is just keeping good backups.
> What I've found in practice is that generally the MTBF (mean-time 
> between failure) of most consumer-grade drives is in the order of 
> years.  When they do finally fail, the drive technology is so far 
> ahead of where it was, that an upgrade is almost always driven by the 
> price-point of current hardware.
> So for non-critical machines where "Real" RAID can't be purchased, you 
> end up no worse off to buy a pair of cheap bigger drives, reload, and 
> restore backups... and now you have three drives (one smaller than the 
> other two) and you usually didn't take that much more downtime than 
> messing around rebuilding the software RAID.
> So I've kinda set the rule for my personal systems that:
> 1. Backups of some sort are a requirement... of everything.
> 2. Software RAID doesn't buy me much, other than a lot of wasted time 
> setting it up and fixing it when a drive finally munches itself.
> 3. Hardware RAID is required for machines that shouldn't ever go 
> down... and preferably it should be with hot-swappable discs.
> Finding the funds for #3 will determine if the machine is really "that 
> critical".  :-)
> So instead of tinkering with Software RAID (which *is* fun, I'll 
> admit), nowadays I'd rather use the second big disc for a nice backup 
> scheme over Software RAID, really.
> An even bigger time-waster appears to be "fakeraid" chipsets like the 
> Highpoint's and others.  I spent a couple hours researching that whole 
> mess tonight for a friend who just happens to have one of those 
> (crappy) cards.  I realized that in order to recover from a failure on 
> his box he's building, since he's not a Linux guru type, we'd have to 
> come up with a huge document that described how to rebuild the machine 
> and get the "fakeraid" card working again, just to see his data if he 
> ever had major problems.  Ick.
> I think he'd be better off buying a good 3Ware card and having two 
> days of his life back to do better things with.  :-)
> Nate
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