Linked by Thom Holwerda on Mon 18th Jan 2010 16:57 UTC, submitted by wanker90210
Hardware, Embedded Systems ACM's latest journal had an interesting article about RAID which suggested it might be time for triple parity raid. "How much longer will current RAID techniques persevere? The RAID levels were codified in the late 1980s; double-parity RAID, known as RAID-6, is the current standard for high-availability, space-efficient storage. The incredible growth of hard-drive capacities, however, could impose serious limitations on the reliability even of RAID-6 systems. Recent trends in hard drives show that triple-parity RAID must soon become pervasive."
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What about 5-way RAID?
by Milo_Hoffman on Mon 18th Jan 2010 20:28 UTC
Milo_Hoffman
Member since:
2005-07-06

I have an idea, how about adding just a little more RAID?


I got a great idea.

Lets write an extra copy of every write somewhere....and the cool things with this design is that reads will be TWICE as fast since the data exists in two different places, also its even more secure than RAID7 because you can loose not just 2or3 disks but up to ONE HALF of all your disks and still have your data.


I should patent this idea.


/seriously at some point it just makes more sense to standardize on RAID 10 doesn't it?

The simplicity of the algorithms of the failure modes for coding in the storage devices alone is less complex/more reliable.

Edited 2010-01-18 20:30 UTC

Reply Score: 2

RE: What about 5-way RAID?
by jwwf on Mon 18th Jan 2010 21:43 in reply to "What about 5-way RAID?"
jwwf Member since:
2006-01-19

I have an idea, how about adding just a little more RAID?


I got a great idea.

Lets write an extra copy of every write somewhere....and the cool things with this design is that reads will be TWICE as fast since the data exists in two different places, also its even more secure than RAID7 because you can loose not just 2or3 disks but up to ONE HALF of all your disks and still have your data.


I should patent this idea.


/seriously at some point it just makes more sense to standardize on RAID 10 doesn't it?

The simplicity of the algorithms of the failure modes for coding in the storage devices alone is less complex/more reliable.


I'm starting to think that 3 way mirroring is not crazy, like the article says:


For the same reasons that make triple-parity RAID necessary where RAID-6 had sufficed, three-way mirroring will displace two-way mirroring for applications that require both high performance and strong data reliability.


In the ZFS context I can see the value of 3 way mirroring just because a 2 way mirror wouldn't be able to recover a checksum error during a reconstruction, just as with single parity raid. On the other hand, in most cases I think I would feel just fine with 2 way mirror and a good backup.

Also, looks like it's not just Sun on the triple parity path:

http://www.ntapgeek.com/2010/01/netapp-triple-parity-raid-patent.ht...

Only skimmed the patent, but it seems like this netapp implementation uses only XOR, like RAID-DP I think, which is interesting.

Reply Parent Score: 2

RE: What about 5-way RAID?
by computeruser on Mon 18th Jan 2010 22:11 in reply to "What about 5-way RAID?"
computeruser Member since:
2009-07-21

Lets write an extra copy of every write somewhere....and the cool things with this design is that reads will be TWICE as fast since the data exists in two different places, also its even more secure than RAID7 because you can loose not just 2or3 disks but up to ONE HALF of all your disks and still have your data.

No. You can lose one drive in each pair of mirrors. A two-drive failure will result in data loss if both mirrors in a pair happen to fail. A two-drive failure in standard RAID6 does not result in any data loss.

Assuming 1 TB drives in a 4 TB array, 1E-15 bit error rate: Here are roughly calculated probabilities of data loss during rebuild after a single drive has failed and been replaced:
mirror (8 drives total): .00859
two mirrors (12 drives total): .00007
single parity (5 drives total): .03436
double parity (6 drives total): .00118
triple parity (7 drives total): .00004
quad parity (8 drives total): 1.6E-9

Reply Parent Score: 1

RE: What about 5-way RAID?
by gilboa on Mon 18th Jan 2010 23:09 in reply to "What about 5-way RAID?"
gilboa Member since:
2005-07-06

seriously at some point it just makes more sense to standardize on RAID 10 doesn't it?


You're wrong.
A. A theoretical RAID7 running only wastes 3 drives - no matter how many drives you have. (15% in 20 drive array). RAID 10 will -always- waste at-least 50% of the space.
B. RAID7 can reliably survive the loss of 3 drives. RAID 10 can only reliably survive the loss of one drive. If you lose two (a full mirror), you lose everything. (The top level strip set is lost)

- Gilboa

Reply Parent Score: 2

RE[2]: What about 5-way RAID?
by Soulbender on Tue 19th Jan 2010 16:20 in reply to "RE: What about 5-way RAID?"
Soulbender Member since:
2005-08-18

RAID 10 can only reliably survive the loss of one drive.


Uh, no. That's not how RAID 10 works. Since it's a stripe of mirrors you can lose all but one drive in each mirror set. How many actual drives that translates to depends on how many drives are in each mirror set and how many mirror sets you have.

Reply Parent Score: 2