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dave johnson | 16 Jun 2006 00:23
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Re: RAID4 ?

On this note, does anybody have a line on a good resource or forum for discussing the RAID4 implementation within the Linux MD driver?  I don't even know if has the capability of running in a VTL-optimized RAID4 configuration.
 
For those that don't get it...
 
Let's say you have 3 disks with disk 3 designated as the sole parity drive in a VTL-modified RAID4 configuration.
 
Let's say that all 3 drives begin as blank and have a capacity of 4 bits and you want to write the following byte of data to the array: 10100011
 
The algorithm requires a read from the parity drive, a parity calculation based on the new data to be written, writing the parity back to the parity drive, and a write of the actual data to the first available data drive.
 
In this scenario, the following would happen:
 
The parity for the capacity to be written to drive one is read from the parity drive (initially blank):
 
0000 (parity drive spins up)
 
The parity is calculated for the first drive write (let's say using XOR):
 
1010  XOR  0000 = 1010
 
Parity is written to the Parity drive:
 
1010
 
Data is written to the first drive:
 
1010 (first drive spins up)
 
Parity is read for the write to the second drive:
 
1010 (still spinning)
 
Parity is calculated for the second drive write:
 
0011 XOR 1010 = 1001
 
Parity is written to the parity drive:
 
1001 (still spinning)
 
Data is written to the second drive:
 
0011 (drive spins up)
 
(after a certain level of inactivity, all drives spin down again)
 
The End.
 
So you see, this modified algorithm for RAID4 implementation is optimized for sequential or near-line usage since only the drive being written to and the parity drive need to be spinning, at the expense of no longer benefiting from the simultaneous spanned writes available in the typical RAID4 algorithm.
 
-=dave
----- Original Message -----
Sent: Thursday, June 15, 2006 2:49 PM
Subject: RAID4 ?

What is the likelyhood of OF supporting RAID4 using the underlying software RAID functionality ?
 
The reason I ask is that in a nearline disk-to-disk target system, RAID4 will allow drives not in use to be powered-down so that only the current active drive and the parity drive are active.  In the event of a parity drive failure, of course all other disks will require spinning up, which may be a problem with a system not designed as a VTL target.   Most systems use a mirror set to store the parity information to get around this limit.
 
It would be a very nice option for those using OF in a home setting or those in an enterprise setting with extreme power and heating constraints.
 
Regardless, what's your guys thoughts on this?
 
Thanks,
 
-=dave
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