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Raid 5 used for Windows 2003 R2 OS disk

Posted on 2010-09-07
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We have a couple servers using hardware Raid 5 on the OS disk.  Microsoft specifically says system and boot files should not be used on this disk.  How bad of a problem is this?  and what is the possibility of having a problem with the restoration of backups?  
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Question by:crtdatacenter
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Lee W, MVP earned 500 total points
ID: 33621588
You CANNOT use a software RAID 5.  That said, hardware RAIDs are fine.  The OS has no way of knowing what kind of hardware RAID is in place.  RAID 5 is used all over the place (in hardware form) and plenty of servers run with it without any problem.  Can you post your reference in Microsoft's statement?
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by:PcGod718
ID: 33621758
the main reason most people dont like the OS on a raid 5 is simply for recovery options.

the "standard" setup is the OS on a RAID 1 mirror setup and everything else on a RAID"x"  x being whatever you want, 5, 6, 10, etc.

this way, the OS is always separated from the data and its an easier recovery option.  If you need to upgrade drives or whatever, its almost always for the DATA drives, not the OS.. well if the OS is living on the data RAID 5, how do you quickly upgrade the drive??
if the OS crashes, how do you quickly get access to the data if everything is on the raid 5?

That said, its no big deal these days to keep everything on a RAID 5, you just need to understand that recovery is just a bit more tedious when setup this way.  
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by:crtdatacenter
ID: 33621808
I saw this in the troubleshooting section of http://support.microsoft.com/kb/323434.  I cannot see where this article makes a distinction between hardware or software RAID.  Your answer seems obviously correct, but is there a Microsoft reference?  I have been looking, but have not found it yet.
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by:Lee W, MVP
ID: 33621907
It's not clearly stated, but that's referring to software RAID.  This can be clearly inferred from this statement (most of the second paragraph):
> In products in Windows Server 2003, parity is a calculated value that is used to reconstruct
> data after a failure. RAID-5 volumes stripe data and parity across a set of disks. When a disk
> fails, Windows Server 2003 uses the parity information to re-create the data on the failed disk.

In a hardware RAID, Windows Server 2003 has no idea and would not be responsible for using parity information to re-create data on the failed disk.


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by:Lee W, MVP
ID: 33621930
Another clue:
> All disks involved in the RAID-5 volume must be dynamic disks.
This is true of SOFTWARE RAIDs, but not hardware RAIDs since the RAID controller handles the RAID and provides the "logical" or "virtual" disk to the OS.
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Author Closing Comment

by:crtdatacenter
ID: 33621961
Yes, good points about calculating parity and dynamic disks.  It is amazing to me though that this is not made more clear given how critical the info is.
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