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SubjectRe: Ok, cool. I think I get it now. Reply to this message
Posted bywildcat
Posted on09/24/07 03:53 PM

> So with RAID 5, any two drives go down in an array and you're fucked. RAID 6
> bumps the total amount of parity to two drives total, so it would take three
> drives to go down to hose your setup. They don't have it set up so that you can
> choose how much parity you want, though? I mean, RAID 6 sounds good enough for
> me in terms of redundancy, but there are some people who might want even more.

True, but RAID 5 does well for reasonably large arrays, and RAID 6 can do even larger arrays, not to mention that the more parity used, the more drives you need, so arbitrary parity is probably a useless feature.

> RAID 4, since the parity is all on one disk, 5 drives could fail, and you would
> still be golden as long as one of those isn't the parity disk. On the other
> hand, if the P. drive and any other drive go down it's game over. I'm sure you
> could specify two drives for parity if you wanted to, though.

No. RAID 4 has the same single drive failure tolerance as RAID 5 (i.e., lose any two drives and you don't have enough data to reconstruct what's missing), it's just that the parity is on one drive instead of distributed. The benefit is that if the parity drive fails, the performance isn't really compromised (but only the parity drive... any others fail, and you're reconstructing data). There are no (official) extensions to RAID 4 that give two parity drives like RAID 6 does for RAID 5.

> In terms of speed with 3 drives, you would get three levels of striping with 5/6
> so they would be faster than 4 where you would have two levels.

Yeah, pretty much.

> For capacity, if they're 120 gig drives, you'd have 240 with 4 and 5, 120 with
> 6.

Except that a three drive array with RAID 6 makes no sense. You're essentially using one drive for data, two drives for parity, and while there may be some performance benefit, it probably won't be enough to justify using two-thirds of your array for redundancy. RAID 1 would probably be better.

> I'm rethinking staying with 3 drives. Getting 4 of lower capacity would crank
> out some sweeeeeet speed on 5/6, or give you as much protection as a plate mail
> condom on 4. The paranoid/obsessive compulsive madman part of me says "4 disk
> raid 4 with double parity; ain't nooooo doubt about it!" but it's not like the
> stuff on my drives is particularly volatile, I'm just lazy and don't want to
> burn periodic DVDs, or spend time doing recovery. I would like to have double
> disk fuck up protection though. Most of the time it's not even simultaneous,
> you'll be using your system while waiting for a replacement to come and another
> drive decides to shit its pants. I think RAID 6 sounds like a winner,
> especially if I can score 4 160g drives for a good price.

RAID 4/5 will very likely be more than good enough for a four drive array. RAID 6 is more for "huge" arrays (probably at least 10 disks), where failure rates on drives can possibly make array reliability worse, because there's a good probability that a second drive will fail while another drive is already failed or reconstructing.

> e; actually now that I think about it, RAID 4 isn't really all that good or more
> protective compared to RAID 6, at least at the # of disks and amount of
> redundancy I'm aiming for. Oh well, it was only the retarded part of me
> thinking anyway.

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