Re: URGENT: Need help rebuilding iir RAID5 array with failed drive

Tom Evans wrote:
> On Thu, 2008-06-26 at 06:49 -0700, Garrett Cooper wrote:
>   
>> Hello,
>>       First off sorry for the cross-post. I typically don't do this
>> but this is an important question, so please bear with me. I'm just
>> trying to get more eyes on the subject so I can (maybe) get a reply
>> quicker...
>>       I'm running 8-CURRENT on my machine and it appears that one of
>> the disks in my RAID5 array has taken a nose dive (BIOS recognizes
>> that it exists, but Intel Matrix Manager claims that the disk is an
>> "Offline Member"). After doing some reading it appears that it's
>> kaput, so I need to get a replacement disk to fix this one...
>>       That aside, I need to determine how to rebuild the array in a
>> Unix environment because Intel only provides instructions for how to
>> use their Windows matrix manager. If anyone can point me to some links
>> or provide me with some pointers on how to correct this issue, I'd owe
>> you a lot; in fact the next time you come by Santa Cruz, CA I'll
>> gladly treat you to some beers or something else you might want :)...
>> Linux solutions (if there isn't a proper one for FreeBSD) are valid,
>> as long as the core data remains uncorrupted and I can do what I need
>> to from a LiveCD. I'm just scared to boot up OS and have it do some
>> irrevocable operation like fsck -y and assume parity errors are ok or
>> something along those lines  (I don't remember if I set rc.conf to
>> fsck -y and I know I can change that from single-user mode, but I want
>> to play things conservatively if at all possible) :\...
>>       Filesystem is UFS2 with softupdates of course.
>>       Point proven that I need to backup my data more often :(...
>> TIA,
>> -Garrett
>>
>> PS If replying on the questions_at_ list, please CC me as I'm not
>> subscribed to that list.
>>     
>
> I'm fairly sure that Intel Matrix metadata cant be created/modified by
> fbsd, only read. You should be able to do whatever rebuild operations
> you need in the BIOS I believe, but that would be an offline operation,
> clearly.
>
> I may be mistaken, and the driver can handle array rebuilds for matrix.
> If it can, the command you are looking for is 'atacontrol rebuild ar0',
> see atacontrol(8).
>
> Tom
>   
Please DONT use FreeBSD to rebuild this RAID5 unit. FreeBSD will detect 
it as a raid 5 Unit and even allow you to use it, add the new disk to it 
and even let you rebuild the raid.
However the ata driver does not have support parity for Raid5. From 
ataraid(4):

     RAID5 is not supported at this time.  Code exists, but it neither uses
     nor maintains parity information.

You can confirm this in code, just look for the Swedish comments in 
ataraid.c
Hence writing to the raidset will give you a glorified raid0.

Cheers,
    Benjamin