If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
Raid 6 in use ? - Reactivation
In July 2004 there was a thread on comp.arch.storage on this - see eg:
a href= "http://groups.google.com/groups?threadm=GoudncugpcwXwmnd4p2dnA%40csd.net&rn um=32&prev=/groups%3Fq%3DRaid%2B6%26start%3D30%26hl%3Dpl%26lr% 3D%26group%3Dcomp.arch.storage%26scoring%3Dd%26sel m%3DGoudncugpcwXwmnd4p2dnA%2540csd.net%26rnum%3D32 " Previous thread on iRaid 6 in use/i/a unfortunately there was no single real use (experimental data or case study) on this, I'd like to ask again about it. Any (non-theoretical) opinions, expertises, successful (or not) stories etc. ??? It seems that more and more low cost sata-to-scsi/fc boxes tries to offer this solution ... And even recently I found Intel's pleople saying that Raid 6 will replace raid 5 : a href="http://www.intel.com/technology/magazine/computing/RAID-6-0505.htm" Intel goes for Raid 6/a So please share your EXPERIENCE! al |
#2
|
|||
|
|||
In article ,
albatros66 wrote: In July 2004 there was a thread on comp.arch.storage on this - see eg: a href= "http://groups.google.com/groups?threadm=GoudncugpcwXwmnd4p2dnA%40csd.net&rn um=32&prev=/groups%3Fq%3DRaid%2B6%26start%3D30%26hl%3Dpl%26lr% 3D%26group%3Dcomp.arch.storage%26scoring%3Dd%26sel m%3DGoudncugpcwXwmnd4p2dnA%2540csd.net%26rnum%3D32 " Previous thread on iRaid 6 in use/i/a unfortunately there was no single real use (experimental data or case study) on this, I'd like to ask again about it. Any (non-theoretical) opinions, expertises, successful (or not) stories etc. ??? It seems that more and more low cost sata-to-scsi/fc boxes tries to offer this solution ... And even recently I found Intel's pleople saying that Raid 6 will replace raid 5 : a href="http://www.intel.com/technology/magazine/computing/RAID-6-0505.htm" Intel goes for Raid 6/a So please share your EXPERIENCE! al Zeroest: In the good old days the RAB standardized the use of the terms RAID-1 through RAID-5, with RAID-0 having become an accepted term for non-redundant layouts (striping). Then it standardized RAID-6 as the accepted term for a variant of RAID-5 with double parity. Problem is that there are several different ways to implement RAID-6, which are highly non-equivalent. The ones commonly labelled RAID-6 seems to be either PQ-parity, or the EVENODD scheme described in the Menon/BBB papers (the three other authors all have names that start with B). Furthermore, today there are many more interesting redundancy schemes that have similar or better space efficiency than RAID-6, and similar or better other properties (real-life resiliency to correlated failures, performance, difficulty of implementation). So while I agree that multi-failure redundant data layouts are the thing of the future, it is not clear that the thing traditionally called "RAID-6" will have significant impact. (Side remark: How many oldies remember the good folks at Storage Computer and their attempt to make RAID-7 a trademark for RAID-4 with cache? Their lawsuit against Hitachi, claiming that they had a patent on all parity-based RAID? They seem to have gone under, which would be well deserved. They did act as if they were a bunch of thieves, liars and crooks, but there may be more to the story.) First: I would guess that those of us that work for a corporation which builds RAID arrays will not discuss our experiences in public, other than in oblique references. The reasons are obvious: The implementation techniques, target markets, and performance/cost tradeoffs between RAID-n (for various values of n, not limited to 1...6) are difficult, multi-faceted, and very important trade secrets. Second: The academic literature has pretty much left research on data layouts as trivial as RAID-n behind. The thrust of research today is in more interestingly complex data layouts (for an example of an interesting, complex, and probably pointless system, see the Oceanstore project at Berkeley). This means that looking at the published research literature will not give too many clues on what is really implemented in the field in disk array. Still, a good study of data layout papers by authors with an industry affiliation will give some insights. Third: Without wanting to name names, I happen to know that there is at least one product line from a major vendor out there (the product may have been cancelled in the meantime, it existed a few years ago) that uses a traditional RAID-6 layout (either PQ-parity or EVENODD, don't remember). But for marketing reasons, the RAID layout was not called RAID-6, because the term "RAID-5" has such a strong brand recognition (SAN administrators "know" that RAID-0 means no fault-tolerance, RAID-1 means fault tolerance with good performance but at a high cost, and that RAID-5 is a compromise between cost and performance, suitable for some workloads, all these statements are oversimplifications). Therefore, the product was marketed as something like "enhanced RAID-5" or "RAID-5++". The marketing team feared that calling it "RAID-6" would scare off customers, who are often quite innovation-shy. -- The address in the header is invalid for obvious reasons. Please reconstruct the address from the information below (look for _). Ralph Becker-Szendy |
#3
|
|||
|
|||
In article , albatros66 wrote:
unfortunately there was no single real use (experimental data or case study) on this, I'd like to ask again about it. Any (non-theoretical) opinions, expertises, successful (or not) stories etc. ??? Probably not what you're looking for, but search the archives of either linux-kernel or linux-raid. The 2.6 kernel has a software implementation of RAID 6 that I do believe some folks are using in production. Also, for an internal product, the new Areca SATA RAID controller claims to do RAID 6. -- Joshua Baker-LePain Department of Biomedical Engineering Duke University |
#4
|
|||
|
|||
|
#5
|
|||
|
|||
On Fri, 20 May 2005 22:15:12 GMT, Curious George wrote:
On Thu, 19 May 2005 17:21:16 -0000, wrote: snip So please share your EXPERIENCE! snip I can only share the obvious generalization that RAID 6 brings additional complexity & parity overhead to raid 5 - which has potential & often real penalty, esp during degredation/rebuilding. The quality of an implementation (as was noted - whose details are usually obscured) would seem quite important. IMHO making the business case is very often not as clear cut as with other currently viable levels. I cannot comprehend it in the low-end enthusiast market - where it is presently being very hyped-up. Raid 6 for the low end enthusiast market? Sheesh, that *is* hype. I don't think there is an actual definition of Raid 6 yet but I haven't looked at standards either. Third: Without wanting to name names, I happen to know that there is at least one product line from a major vendor out there (the product may have been cancelled in the meantime, it existed a few years ago) that uses a traditional RAID-6 layout (either PQ-parity or EVENODD, don't remember). But for marketing reasons, the RAID layout was not called RAID-6, because the term "RAID-5" has such a strong brand recognition (SAN administrators "know" that RAID-0 means no fault-tolerance, RAID-1 means fault tolerance with good performance but at a high cost, and that RAID-5 is a compromise between cost and performance, suitable for some workloads, all these statements are oversimplifications). Therefore, the product was marketed as something like "enhanced RAID-5" or "RAID-5++". The marketing team feared that calling it "RAID-6" would scare off customers, who are often quite innovation-shy. Some vendors (NetApp) are calling it Raid DP, not double parity but rather diagonal parity. Don't ask me how it works because I have no clue, I just know that some pretty smart people say it's incredibly more safe than non-DP parity raid. ~F |
#6
|
|||
|
|||
In article ,
Faeandar wrote: Some vendors (NetApp) are calling it Raid DP, not double parity but rather diagonal parity. Don't ask me how it works because I have no clue, I just know that some pretty smart people say it's incredibly more safe than non-DP parity raid. I think a few people from NetApp gave a talk about a new parity-scheme at one of the USENIX FAST (File And Storage Technologies) conferences in San Francisco, a little while ago. Only thing I remember is that it was either early in the morning or right after lunch, so I had a hard time staying awake, but the talk seemed very interesting. Grep the FAST proceedings for "NetApp" "Parity" "RAID" or such (since there have been only 4 or so FAST conferences, a visual grep will probably suffice). I'm at home now, and the proceedings are in the office, and I don't feel like doing this online over a low-speed link. Reminder: The fact that a few researchers from NetApp give a talk at a research conference doesnt mean that NetApp ships exactly that idea in a product. -- The address in the header is invalid for obvious reasons. Please reconstruct the address from the information below (look for _). Ralph Becker-Szendy |
#7
|
|||
|
|||
|
#8
|
|||
|
|||
Zak wrote:
wrote: Reminder: The fact that a few researchers from NetApp give a talk at a research conference doesnt mean that NetApp ships exactly that idea in a product. Diagonal parity sounded like a simple enough idea to actually work. But obviously there are implementation details... IIRC it is relatively simple, and somewhat elegant. It involves a normal RAID-5-style set of stripes, each of which includes an additional segment used for the 'diagonal' parity that does not otherwise participate in the stripe. So when a single disk fails, data is rebuilt exactly as it is in a normal RAID-5 configuration. If a second disk fails, then the 'diagonal' parity comes into play. It's generated by XORing segments from successive stripes (on successive disks), such that it's always possible to regenerate one of the two missing segments in any normal RAID-5-style stripe from information in surrounding stripes (plus the diagonal parity) - and once you've done that, the other missing segment can then be regenerated by the normal RAID-5 mechanism. While with a single disk failure degraded performance is therefore similar to a normal RAID-5 array, with a double failure this two-stage approach to reconstructing each stripe slows things down to a crawl: far better than losing your data entirely and having to reconstruct it from backup material, but in many cases not adequate for acceptable continuing performance in a production environment. There's also of course additional write overhead during normal operation, but NetApp may be in a better position than most to tolerate this because of their ability (due to the NVRAM that's an integral part of their server architecture plus their 'write-anywhere file layout') to collect dirty data and dump it to disk in full-stripe or even multi-stripe batches where they can just blind-write the parity information along with it rather than reconstruct it with multiple disk accesses as is necessary for small write operations. - bill |
#9
|
|||
|
|||
Bill Todd writes:
If a second disk fails, then the 'diagonal' parity comes into play. It's generated by XORing segments from successive stripes (on successive disks), such that it's always possible to regenerate one of the two missing segments in any normal RAID-5-style stripe from information in surrounding stripes (plus the diagonal parity) - and once you've done that, the other missing segment can then be regenerated by the normal RAID-5 mechanism. I'm having trouble wrapping my mind around that. There are well known ways of doing this kind of stuff with finite field arithmetic but I don't see how to do it with simple xor's. But maybe I don't understand the RAID terminology and that's confusing me. Is there a more detailed explanation online somewhere? Thanks. |
#10
|
|||
|
|||
Paul Rubin wrote:
Bill Todd writes: If a second disk fails, then the 'diagonal' parity comes into play. It's generated by XORing segments from successive stripes (on successive disks), such that it's always possible to regenerate one of the two missing segments in any normal RAID-5-style stripe from information in surrounding stripes (plus the diagonal parity) - and once you've done that, the other missing segment can then be regenerated by the normal RAID-5 mechanism. I'm having trouble wrapping my mind around that. There are well known ways of doing this kind of stuff with finite field arithmetic but I don't see how to do it with simple xor's. But maybe I don't understand the RAID terminology and that's confusing me. Is there a more detailed explanation online somewhere? Thanks. About a minute Googling for raid-dp at www.netapp.com yielded http://www.netapp.com/tech_library/3298.html?fmt=print - bill |
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
IDE RAID | Ted Dawson | Asus Motherboards | 29 | September 21st 04 03:39 AM |
Need help with SATA RAID 1 failure on A7N8X Delux | Cameron | Asus Motherboards | 10 | September 6th 04 11:50 PM |
Asus P4C800 Deluxe ATA SATA and RAID Promise FastTrack 378 Drivers and more. | Julian | Asus Motherboards | 2 | August 11th 04 12:43 PM |
Gigabyte GA-8KNXP and Promise SX4000 RAID Controller | Old Dude | Gigabyte Motherboards | 4 | November 12th 03 07:26 PM |
RAID-1 reliability | marcodeo | Storage (alternative) | 26 | August 30th 03 09:53 PM |