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SubjectQuick answer regarding parity blocks and RAID Reply to this message
Posted bywildcat
Posted on09/24/07 01:16 AM

> I'm going to get a RAID setup soon for redundancy. I was reading about RAID
> 4/5/6, and I guess I don't really understand what the parity blocks are and how
> these RAID modes compare to your typical striped or mirrored setup. I don't
> want to buy more than 3 drives. If I can get more speed as well as the all
> important redundancy then sign me up.

I really only know about RAID 5, since that is the most common of the three you mentioned.

All you need to know about parity is that each block provides a hash of the data in the same block on the other drives. You're basically dedicating one complete drive in the array to parity, but it's spread out among all the drives, so that if one fails, the others have 2/3 of the raw data (for a three drive setup), and won't need to reconstruct the data from parity (which slows things down). Once the failed drive is replaced, the fresh drive can be rebuilt from the raw and parity data on the other drives.

You can do RAID 5 with three drives. This should be almost as fast in reads as a two drive RAID 0 setup. (Wikipedia says almost as fast as a three drive RAID 0, but I'm a little skeptopotamus.)

Apparently, RAID 4 is like RAID 5, only all the parity data is kept on one disk. RAID 6 is RAID 5 only with two parity blocks instead of one (which requires one more hard disk than 4 or 5, but gives protection against double disk failures).

Entire Thread
Subject  Posted byPosted On
*Quick question regarding parity blocks and RAID   VanillaDome09/24/07 00:44 AM
.*Just found out about nested RAID NT fucking sweet  VanillaDome09/25/07 02:24 PM
..Quick answer regarding parity blocks and RAID   wildcat09/24/07 01:16 AM
..*Ok, cool. I think I get it now.  VanillaDome09/24/07 05:16 AM
...*Re: Ok, cool. I think I get it now.  wildcat09/24/07 03:53 PM
....*Thanks for clearing that up  VanillaDome09/24/07 07:34 PM
...*don't forget the raid controller could fail too  jajig09/24/07 05:28 AM
....*To be fair, the drives are much more likely to fail than the controller, though *nt*  wildcat09/24/07 03:54 PM
....*Re: don't forget the raid controller could fail too  VanillaDome09/24/07 06:11 AM