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SubjectQuick question regarding parity blocks and RAID new Reply to this message
Posted byVanillaDome
Posted on09/24/07 00:44 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.




SubjectQuick answer regarding parity blocks and RAID new 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).




SubjectOk, cool. I think I get it now. new Reply to this message
Posted byVanillaDome
Posted on09/24/07 05:16 AM



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.

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.

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.

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

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.

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.




Subjectdon't forget the raid controller could fail too new Reply to this message
Posted byjajig
Posted on09/24/07 05:28 AM



with the low prices of HD drives I just do a raid mirror. It gives me faster read speeds and unless both drives fail at the same time my data is safe. It also has the added bonus of still being compatible with undelete software in case you accidentally delete your porno.






SubjectRe: don't forget the raid controller could fail too new Reply to this message
Posted byVanillaDome
Posted on09/24/07 06:11 AM



If the controller goes out, the data should be safe, right? It wouldn't flip out and write some crazy shit on your disks. You wouldn't be able to seamlessly keep your system going though. With mirror mode, you could just pop in an individual drive and keep it rollin'. That's a good point; you would be down until they sent you a replacement. Is it rare for raid controllers to fail? One solution would be to just keep an extra one on hand. This RAID plan keeps getting more expensive...

Too bad tape drives and media are so expensive. It would be nice to just have it automatically schedule your backups every night.

e; I'm looking at some of the prices for RAID controllers on Newegg. Hole E. Shit.


SubjectRe: Ok, cool. I think I get it now. new 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.




SubjectTo be fair, the drives are much more likely to fail than the controller, though *nt* new Reply to this message
Posted bywildcat
Posted on09/24/07 03:54 PM



> with the low prices of HD drives I just do a raid mirror. It gives me faster
> read speeds and unless both drives fail at the same time my data is safe. It
> also has the added bonus of still being compatible with undelete software in
> case you accidentally delete your porno.
>





SubjectThanks for clearing that up Reply to this message
Posted byVanillaDome
Posted on09/24/07 07:34 PM



So a single parity block can't reconstruct data on its own, you need other data as well.




SubjectJust found out about nested RAID NT fucking sweet new Reply to this message
Posted byVanillaDome
Posted on09/25/07 02:24 PM



0+1 here I come




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