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Raid 5 array size misreported

Posted on 2006-04-03
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Running four SCSI 73 G HDD's in a Windows 2000 server for data, with Raid 5 configuration via Win 2K Server's disk manager.  We had a disk fail, and it was replaced.  We rebuilt the array, Disk Manager shows all four disks "online" and working fine, but the data only shows as 205 gig and not 280 gig or more, which it should with four disks.  I'm at a loss; all four disks are shown as one drive as they should be, the sysinfo file shows all four disks correctly but still shows the volume as 70+ gig short.

How do we repair this?
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Question by:Smugdad
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garycase earned 500 total points
ID: 16365595
A Raid-5 array with 4 disks will have a capacity equal to 3 of the disks ==> one disk is used for the parity information;  without the redundancy you would not be able to lose a disk without losing data.

Your size is absolutely correct -- you are mistaken in expecting 280gb.
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by:jhance
ID: 16365795
Perhaps you were not running RAID 5 prior to the failure??  It's not clear that you rebuilt the array or "rebuilt" from a backup.  But a RAID 0 would have 280GB or so.  But, as garycase noted, a RAID 5 will have (73 * 4) - 73 GB of space in the array.  That should be about 220GB but, as you know, 73 GB is not necessarily 73GB...
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by:garycase
ID: 16365875
"... 73 GB is not necessarily 73GB... " ==> well, 73GB is 73GB, but you do have to consider whether you're speaking "hard-disk-ese" or "computer-ese".   In "computer-ese" a GB = 1024x1024x1024; whereas in "hard-disk-ese" a GB = 1,000,000,000.   So a 73GB disk (as measured by the hard disk makers) will report 67.99GB in Explorer (which reports in "computer-ese").   So three of them will report 203.96GB.   The size of your disks is probably not EXACTLY 73GB -- so you're seeing 205GB instead of 203.96GB ==>  but as I noted, the size is absolutely correct for a RAID-5 array.

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by:garycase
ID: 16387131
Smugdad -- did you have any further question about this?   In case you'd like to become more familiar with the various RAID levels, a quick "Google" on "RAID levels" will provide you with plenty of references of varying degrees of technical detail.   This is a fairly nice one that shows the most common levels pictorially:
http://www.prepressure.com/techno/raid.htm
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by:Smugdad
ID: 16388204
Thanks, everybody!  I guess I just assumed (which has in it "ass" "su(e)" and "me") that 73*4 should be somewhere between 280 and 300.  My mistake.  Thanks to all of you for straightening me out!
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by:garycase
ID: 16392228
You're welcome -- don't forget to close the question ("Accept" an answer or split the points, as you see fit)
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by:garycase
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Smugdad -- just noticed this was still open.   Don't forget to close it :-)   ("Accept" an answer or split the points, as you see fit)
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