Talk:Standard RAID levels

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Admittedly, this page is likely to be accessed only be those with some prior knowledge of the subject, but even so, the page ought to begin with a more substantial introduction to orientate the reader more clearly to the subject, in the manner of an encyclopedia entry. R.braithwaite 20:47, 30 December 2006 (UTC)

Granted this article is very lacking in an introduction, but doesn't RAID cover the basics to what RAID is? Cburnett 06:23, 31 December 2006 (UTC)

The article was split off in the first place because the original article was 213213246513 kb long! Sheesh. The whole point was to cut them down, not make each one as long as the last. Summarizing the section from the orig. article should be adequate.// 3R1C 15:20, 25 January 2007 (UTC)

I whole-heartedly disagree. Each RAID level could easily be made into its own article and should be encouraged to do so. Each one made into its own article can then be summarized in a paragraph on this page with a {{main}} link to the article. Cburnett 15:35, 25 January 2007 (UTC)

I've added a draft for the RAID 4 missing section. Hernandes 21:14, 23 January 2007 (UTC)

Contents 1 JBOD and SPAN 2 Mixing RAID modes 3 Typo in the RAID-5 section? 4 RAID-6 Resource 5 left-symmetric and left-asymmetric

[edit] JBOD and SPAN

Some tells that JBOD and SPAN is the same. Other says that JBOD is not RAID. JBOD will every disk have a volume and SPAN vil all disks be like one big disk. 80.203.188.154 15:43, 8 February 2007 (UTC)

Agreed. I have not come across any vendor of SAN attached RAID controller that provides spanning and calls it JBOD. If you create a JBOD from a physical disk, it will be presented as a single logical disk that corresponds directly with ONE physical disk. I first heard the term JBOD in IBM in 1995 and it was used to mean a single disk that was not being managed by the RAID controller. Can anyone provide examples of vendors that are allowing spanning / concatenation and call it JBOD - I know of none. I think a clarificaiton that JBOD was originally used to mean a single physical disk that is presenting a single logical disk that corresponds 1:1 LBA wise. Baz whyte 20:19, 26 February 2007 (UTC)

[edit] Mixing RAID modes

Is it possible to mix RAID modes? I remember reading about RAID 1+0. Is it possible to use RAID 0 with any other mode? It seems like you'd have twice as many drives to support the addition of RAID 0. (So RAID 6+0 would need a minimum of eight drives?) --72.202.150.92 05:54, 30 January 2007 (UTC)

Absolutely. See nested RAID levels. Cburnett 06:01, 30 January 2007 (UTC)

[edit] Typo in the RAID-5 section?

In the RAID-5 section, it is currently written:

In the example above, a read request for block "A1" would be serviced by disk 1. A simultaneous read request for block B1 would have to wait, but a read request for B2 could be serviced concurrently

Looking at the figure, should it instead say "disk 0"? I hesitate to change it, because I actually don't know a lot about RAID yet.--Skoch3 02:58, 1 February 2007 (UTC)

Also, same with RAID-4 section.--Skoch3 03:04, 1 February 2007 (UTC)

Yes and corrected. Cburnett 03:09, 1 February 2007 (UTC)

Great, Thank you! Am I supposed to delete this section now? Not sure of the custom. --Skoch3 07:56, 3 February 2007 (UTC)

[edit] RAID-6 Resource

I personally found the description of RAID-6 presented here to be rather lacking on how the double parity works. A PDF at http://www.infortrend.com/3_support/pdf/RAID6_White%20Paper.pdf seems to go into a more detailed explanation, but I'm not informed enough to add it here. Maybe someone with more knowledge would be able to help? If nothing else, it is a nice resource for others.Quad341 20:18, 27 February 2007 (UTC)

[edit] left-symmetric and left-asymmetric

While setting up a RAID system recently, I wondered whether I should choose the "left-symmetric" vs. "left-asymmetric" option.

A quick Google led me to "Left-symmetric offers the maximum performance on typical disks with rotating platters." [1] which answered my immediate question, but didn't really explain what it is. Later I found an illustration that seems to explain the difference: "Left-symmetric and left-asymmetric algorithm are demonstrated in Figures" [2].

Using our current Wikipedia notation, "left-symmetric" and "left-asymmetric" look like this:

left-asymmetric:

 A1 A2 A3 Ap
 B1 B2 Bp B3
 C1 Cp C2 C3
 Dp D1 D2 D3
 E1 E2 E3 Ep

left-symmetric:

 A1 A2 A3 Ap
 B2 B3 Bp B1
 C3 Cp C1 C2
 Dp D1 D2 D3
 E1 E2 E3 Ep

If I understand correctly, our RAID 5 illustration currently shows the "left-asymmetric" variant, which (if I understand correctly) no one really uses -- everyone uses "left-symmetric" because of its greater performance.

(By inspection, I can see that "left-symmetric" has better performance, because if you read any 4 consecutive blocks -- for example, if you read A2, A3, B1, B2 -- left-symmetric can read them all simultaneously from all 4 disks, while left-asymmetric maps 2 of them to the same disk -- in this case, A2 and B2 -- so the read from B2 must wait until A2 is done).

Could someone confirm for me that everyone that uses RAID5 really uses "left-symmetric"? (And if not, why would anyone use anything else?)

Could someone update the RAID-5 section to illustrate the type of RAID5 that, in practice, everyone uses?

--68.0.120.35 01:29, 11 March 2007 (UTC)