RE: Raid 5 - 7 drives as good as 8?

[Steve Morrison <smorrison@xxxxxxxxxxxxx>]

Jennifer 

I think you'll find that the size on your existing drives will change, if
you add drives to your parity set. You currently have 6 drives in the set,
all the same model, with the parity data spread across 4 drives. If you
subtract 3145 MB (the drives that hold the parity information) from 4194 MB
you get 1049 MB. Multiplying that by 4 gives 4196 MB gives the space
required to store the parity information (with overhead and rounding).

If you move up to 8 drives in the set, you'll probably see the reported size
on all drives change to 3669 MB. It should never take more than 1 disk of
information to store the parity data. It is just being spread across a
larger number of drives to improve performance. 

Steve Morrison
Beacon Insurance
Phone 940-720-4672
Fax   940-720-4345


-----Original Message-----
From: Jennifer Wilson [mailto:jennifer.wilson@xxxxxxxxxxxxxxxxxx] 
Sent: Friday, July 23, 2004 8:35 AM
To: midrange-l@xxxxxxxxxxxx
Subject: Raid 5 - 7 drives as good as 8?

Hi,

Trying to get my head round installing some more drives on our Model 170.

Currently has 6 x 4GB drives in RAID 5 with parity spread across 4 drives

so -

              Size    %  
 Unit  Type    (M)  Used 
    1  6607   4194  80.5 
    2  6607   3145  80.3 
    3  6607   3145  80.8 
    4  6607   3145  80.7 
    5  6607   3145  80.3 
    6  6607   4194  80.3 

I have 2 more 4GB drives waiting to go in but I'm unsure as to whether when
I add the 8th drive will parity automatically get spread across all 8 drives
rather than 4?

If so that means with 7 drives I get 25.1GB (4 drives at 3145 and 3 drives
at 4194 utilisation)
and with 8 drives I get 25.1GB (8 drives at 3145 utilisation)

Is this correct?

Thanks,

Jennifer.

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