Maximum Nos of Disks in RAID 6 Set

H

haileris

Limp Gawd
Joined
Jun 6, 2010
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458
Hi

With all the mega large disk enclosures in play on this forum I was wondering if anyone has the RAID-6 maximum of 16 disks and if so how long it took to initialise the drive and if heaven forbid one disk (or two) failed how long it took to rebuild the RAID set.

I'm intending to build another disk enclosure shortly using Hitachi non-Enterprise 2TB SATA disks - still unsure whether to go to 12 x 2TB or 16 x 2TB.

Cheers
D
 
RAID 6 does not have a maximum of 16 disks. Initialization and rebuild speed are dependent on the controller you use.
 
As Blue Fox says, the time is dependent on the controller, among other things.

But you can at least put an approximate lower limit on it. Say the drives can read and write at about 100 MB/s (the fastest 2TB drives may be a little faster, but lets just use the round number). If all of them are read and written to perfectly in parallel, and the bottleneck is the drive speed, then you have 2 TB @ 100 MB/s = 20,000 sec = 333 min = 5.56 hours.

As an empirical data point, I have a system with an Areca 1261ML controller and 15 WD RE4-GP drives in RAID 6. It can initialize in about 9.5 hours. A rebuild after losing one disk takes longer, around 20 hours. If I turn off the individual HDD write caches, then both of those times go up by a factor of about 5.
 
John - many thanks, I was really looking for some "real life" examples as opposed to the theoretical munge that vendor's claim!

Blue Fox - to be fair I nearly put "alledgedly" in that RAID 6 maximum but Adaptec FAQ's clearly state a maximum number of disks in a parity set. Whether that is their maximum based on how they calculate parity or merely their best practice I personally don't know.

http://ask.adaptec.com/scripts/adaptec_tic.cfg/php.exe/enduser/std_adp.php?p_faqid=15135


Many thanks
D
 
That adaptec page is for the slightly older 3xxx series cards but practically you can go over 16 drives. Personally I get nervous at about 12 but to each their own.
 
There is a slight risk to increasing the number of disks beyond a certian point. The trouble happens if you do a rebuild and during the rebuild 2 other drives get kicked out of the array because they have unreadable sectors.
 
Thanks guys - yeah I get nervous after about 12 but that isn't based on any technical or scientific fact, just a pessimistic thought process! If 20 people lept on this thread saying I run 16 drive RAID6 sets no problem 4 years of uptime etc it would guide me to run with that config but without that real life confidence check I'll probably stick with 12 as my rule of thumb.

Btw, that 16 drive limit is in a few other places on the Adaptec site - for example http://www.adaptec.com/en-US/_common/compatibility/_education/RAID_level_compar_wp.htm but I haven't seen that quoted anywhere else.

I guess on the readable sectors issue I'm not as concerned as I would still be able to get data off the disk during the rebuild process (though I intend to back it up so there shouldn't be a recovery issue). Of course if I am wrong in this assumption please advise!
 
Over some point you should investigate raid7 (if available), raid50 or raid60.
 
I have been running a 20 disk raid6 array for quite some time. I recently upgraded to 2 TB disks. I haven't had to rebuild since upgrading to 2 TB disks but it took about 9 hours for 1 TB disks.
 
I'm running 2 x 20 drive RAID 6 arrays myself. Drive limits are only imposed by controllers (mine limits me to 32 drives in RAID 6 for example). RAID 6 has a minimum number of drives (4), but no theoretical maximum. If you're looking for a RAID card though, I wouldn't recommend Adaptec a whole lot.
 
drescherjm said:
Over some point you should investigate raid7 (if available), raid50 or raid60.
Click to expand...

It would be next to impossible to find a controller that uses RAID 7 as it was a proprietary RAID-level used by Storage Computer Corporation (out of business) and IIRC it was only a cached RAID3 so im not sure it has any advantage over RAID6 anyway.
 
Thanks a lot guys.

Blue Fox - out of interest what drives are you using, Enterprise SATA or economy class ones?

I currently have three HP P800's and two HP P400's (I can get them cheap!) but going to buy some sort of Areca RAID card for this project. Worst case I would use a P411 but pretty miffed that invoking RAID-6 requires an expensive license upgrade.

Cheers
D
 
nitrobass24 said:
It would be next to impossible to find a controller that uses RAID 7 as it was a proprietary RAID-level used by Storage Computer Corporation (out of business) and IIRC it was only a cached RAID3 so im not sure it has any advantage over RAID6 anyway.
Click to expand...

Thanks. I thought it was triple parity raid. Although its been years since I have looked into that..
 
Like pretty much everyone else here, I'm using 2TB Hitachi drives. They're just the regular desktop versions as the enterprise ones are more than double the price.
 
Thanks all, certainly good to know others running signficant RAID 6 configs reliably using consumer disks.
 
20x 1tb (seagates desktop drives) in raid 6 on areca 1280 for ~ 18 months, no problems.
Had to replace 1 drive i think rebuild was ~ 14 hours
 
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