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RAID 5 for combined OS and Data Volumes....Bad Idea?

Posted on 2009-07-15
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Last Modified: 2013-12-05
Quick synopsis
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Basically I work for a company which provide software which requires a database servers.  The standard set-up for these server's is always RAID 5 using some like 5 physical disks and 1 hot spare.  

We then partition a C and D drive across the RAID container, OS on C: drive and the data on D: drive

I encountered an issue the other day when one of the disks on the RAID failed - Windows blue screened and crashed.  On trying to reboot the server reported "Unable to find bootable device" which would suggest no MasterBootRecord could be found.

We then added the hotpspare to the RAID5 and let it start rebuilding from parity bits.  Whilst it was rebuilding  at got to around 7% and 10% rebuild, we tried rebooting and received the same "Unable to find bootable device".  We eventually waited for the disk to fully rebuild and then Windows would boot up with no problems.

The Questions
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Now, having the OS volume and the data volume on the same RAID is normally bad idea in my opinion....Would you agree and that RAID 1 for the OS and RAID 5 for the data volume would be better and if so, why?

I presume the OS crashed and wouldn't load because elements of the OS were held on this faulty disk which failed, and would only boot when it had rebuilt and could access what is needed again...however isn't RAID 5 supposed to provide this data using parity bits whilst the disk is faulty or rebuilding?

Ben
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Question by:benowens
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5 Comments
 
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Accepted Solution

by:
andyalder earned 750 total points
ID: 24857890
RAID 1 for the OS and RAID 10 for the data is preferable, it may cost more but it's much faster. RAID5 performance is poor fir write and in your current single RAID 5 you've got the pagefile on RAID5 which is always advised against.

RAID 5 performance with a failed disk can be so slow that the server's unusable.

The OS crash shouldn't be down to the single disk failed in the RAID but you may have encountered an unrecoverable read error on one of the remaining disks,
 
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Assisted Solution

by:lnkevin
lnkevin earned 750 total points
ID: 24863319
however isn't RAID 5 supposed to provide this data using parity bits whilst the disk is faulty or rebuilding?

It should. Your case, I suspect you have more than one failed disk. Normally, when you have only one failed disk, if you had a hot spare already, it would go ahead and rebuilt with your hot spare (shouldn't need to manually add hot spare. Your case, when you add the hot spare, it rebuilt the second failed drive and your hot spare is not available any more.

I agree with Andy on RAID 1 and 10 configuration since you are running database that requires more write access for logs.

K
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by:benowens
ID: 24868259
RAID 10 is a good point.  I remember seeing this recomendation for Progress databases and guessed it would be a standard for most database systems.  RAID 10 is quite expensive though isn't it?

It was only 1 faulty disk, you could see that in RAID configuration and we only had to replace one disk for sure!

I guess I just wanted someone to confirm that using RAID 5 for an OS partition is stupid because.... if the data in inaccessible on a disk, then obtaining that data from parity bits is going to be slow.  The OS doesn't like that it can't get the information immediately and the OS will crash as a result.  It was only when the data had been recreated from the disk rebuild that the OS was happy again.  Does that sound like a correct summary.  Can someone confirm and back that up if it's true?
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Expert Comment

by:lnkevin
ID: 24869442
It's a fair summary and I believed Andy did mention that as well.

K
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Expert Comment

by:andyalder
ID: 24869535
I wouldn't expect the BSOD, in a boot from SAN environment you're meant to be able to failover paths which can take several seconds, of course the driver parameters tell it this and so the OS knows to wait. So it's possible later drivers will fix the BSOD problem.

During boot though drivers aren't loaded and it's using primitive int13h extensions and boot loaders so I could well expect problems there.
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