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RAID 5 Available Disk Space Ratios

Posted on 2009-04-01
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Last Modified: 2012-05-06
Can anyone tell me why 8 physical 1TB disks would yield 6.34TB of usable space on a Dell NF600 NAS running Storage server 2008 after the RAID 5 process and 3 physical 1TB disks would only yeild 1.8TB usable space after the RAID 5 process?

Did I likely do something wrong or does one get a better yield with more physical disks?  I am NOT an expert with this but I need answeres for a manager who has seen the difference...

Thanks.
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Question by:LTWadmin
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by:Andres Perales
ID: 24043790
What type of RAID are you using, and did you setup the use of Hotspares?
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by:LTWadmin
ID: 24044386
Peralesa:

On the Server:  8 Physical 1TB Disks two logical volumes (20GB for the OS) :  (neither has hot spares from that I can tell)

1) RAID-5  20.00GB  Windows Disk 0  SATA  No Read Ahead  Write Back  64 KB  Enabled
2 RAID-5  6,497.00GB  Windows Disk 1  SATA  No Read Ahead  Write Back  64 KB  Enabled

On the MD-1000 Array Chassis  - 3 Physical 1TB disks:

1) SATA  931.00GB  931.00GB  0.00GB  No  DELL  WDC WD1002FBYS-18A6B0   03.00C06  WD-WMATV119120091200  50022192BCA12544

2) SATA  931.00GB  931.00GB  0.00GB  No  DELL  WDC WD1002FBYS-18A6B0   03.00C06  WD-WMATV119120091200  50022192BCA12544

3) SATA  931.00GB  931.00GB  0.00GB  No  DELL  WDC WD1002FBYS-18A6B0   03.00C06  WD-WMATV119120091200  50022192BCA12544
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Andres Perales earned 500 total points
ID: 24045770
Alright on the MD-1000 Chasis you have 3 - 1TB drives of which on 931 GB are usable, then you set them up in a RAID-5, because it is a RAID-5 you essential take one disk away, that one disk is used for the Parity in the RAID so, 931 multiplied by two gives you 1,862 GB or roughly 1.8 TB.
On the server, are you setting up the RAID on the controller or using windows?  I would say controller so your 8 Drives setup in RAID5 is really is 931 GB multiplied by 7 which gives you roughly 6,517 GB, minus 20 GB for the OS and that gives you 6,497 or roughly 6.4 TB usable for storage.
Hope this made some sense to you!
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by:LTWadmin
ID: 24049434
peralesa - I think you nailed it but before we cap this off you had a question about whether we had setup raid "on the controller or using Windows".  Good question.  I struggled with the setup of the server when it arrived but I do recall using the Dell Systems build and update utility with help from their tech support.  I was negotiating their menu system converting physical drives to logical drives etc.  It requires you to insert the Operating system CD mid stream so I'm thinking it might be on the controller?  
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by:sublifer
ID: 24054714
Its because a terabyte is not 1000GB.  It should be 1024GB but instead manufacturers have made it 1,000,000MB or 1,000,000,000KB (or further up) and when divided by 1024 and again by 1024 to the right point, you get close to that 931GB.  A portion of the drive is alsol used by the File Allocation Table.

Anywho, raid5 works by using the equivalent capacity of one drive from the array and uses that space for parity info that can be used to rebuild a drive if one dies.  The parity info is spread across all the drives, not just all on one. so 3 x 1terabyte drives in raid 5 yields a usable size of 2 terabytes. 6 would give 5 and 8 would give 7.  The ratios improve as you add drives but the speed usually gets slower do to all of the parity calculations.  The best middle ground is usually 4 to 6 drives.  
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by:sublifer
ID: 24054791
in regards to your last question, yes you're using a hardware raid controller.  Software raid is done through the operating system and for obvious reasons you can't have the system partition on a software raid set.

If you ever need to get back into the controller to change your array, watch when the machine boots.  It will probably be a combo like Shift + F7 to get in.
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