Talk:Standard RAID levels
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[edit] RAID-0 performance
The statement about performance of Raid0 in games yielding negligible benefits is incorrect, games regularly access large chunks of data not just during level loads but also during gameplay, Oblivion is a perfect example, the difference between loading times for new area content between Raid0 and non Raid would most likely be more than 20 seconds each time, meaning it's essentially the difference of smooth uninterrupted gaming compared to sitting and waiting for content to load, hardly "minimal". —Preceding unsigned comment added by 203.214.17.51 (talk) 05:19, 8 September 2007 (UTC)
- Get more RAM. One point that seems to come up pretty frequently in these types of discussions is that RAID-0 boosts overall performance when there is a virtual memory bottleneck. However, you can solve this problem with RAM, which is cheaper, more robust, and easier to setup. Afterwards, the virtual memory bottleneck goes away and RAID-0 once again has no measurable effect. Of course, if you have reliable sources to the contrary, share them. Ham Pastrami 09:36, 8 September 2007 (UTC)
[edit] Introduction
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 orient 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've added a draft for the RAID 4 missing section. Hernandes 21:14, 23 January 2007 (UTC)
[edit] technical errors
The RAID-0 section has some errors such as saying that the seek time will be divided by the number of drives if you're reading less than the stripe size. The claim that "reliability of a given RAID 0 set is equal to the average reliability of each disk divided by the number of disks in the set" is also suspect since the reliability cannot possibly be better than the least reliable drive in the array. It also is discussing an arbitrary array and suddenly starts assuming two disks. It sort of looks like someone who didn't understand the existing text added some sentences at the end of a previously good paragraph.
[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)
Moreover, we have this clause:
- Many Linux distributions use the terms incorrectly and refer to JBOD as "linear mode" or "append mode".
Is there any proof that it's incorrect usage of term? I tend to think that basically, there are no well-understood term for that thing. JBOD, Spanning, concatenation, linear and appending can all mean the same thing or different things, depending on who uses it. May be we shoudl reflect that such a mess exists, instead of trying to confuse our reader? --GreyCat 05:08, 6 September 2007 (UTC)
I've never seen this usage of JBOD before. JBOD is the *absence* of RAID of any form, you are just getting direct access to a bunch of disks. Vendors like Adaptec's, Sun's and SuperMicro's usage of the term matches this. It seems to me the notion that JBOD is concatenated RAID-0 is a misunderstanding who has spread out from ordinary PC users. I'd also like to express my skeptisism about calling "linear mode" incorrect. It may be unusual terminology, but I don't see what's wrong with it. FWIW, Solaris Volume Manager avoids the issue and doesn't use the RAID word for "concatentations" and "stripes". Kjetilho 13:31, 12 November 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)
[edit] RAID 6 explanation
The figure used in the explanation of RAID 6 seems to be very confusing as it mentions two parity blocks per row, although the second syndrome is definitely not parity, as explained in the HPA's paper cited below.
MJ 18:12, 29 May 2007 (UTC)
[edit] RAID 5 Performance
The article says that raid 5 performance is nearly as good as a raid-0 array with the same number of disks. This isn't quite accurate.
In theory at least, a raid 5 array should be the same performance as a 1-disk smaller raid-0 array (works out at the same amount of usable storage). Using a 4-disk raid 5 array as an example, every 4th block on the disk would be a parity block, which would need to be skipped when reading. Skipping a block should take the same amount of time as reading it, after all the disk still has to spin the same amount. This means that each disk only spends 3/4 of it's time reading. Multiply this across all 4 disks (parallel reading), it works out at 3x single disk speed, or the same as a 3-disk raid-0 set.
If anyone can verify this it should be added to the page.
85.133.27.147 13:33, 15 June 2007 (UTC)
- You are 100% right about reads but RAID5 always had a lacking performance of writes. When you are writing a small amount of data (less than a full stripe) RAID5 has to: read the old data blocks, calculate parity, write new data blocks and the new parity block. In the disk arrays I've tested (EMC Cxx, IBM DS4xxx, Sun FLX) such writes have practical 50% to 65% performance impact in terms of writes/second. Full stripe writes are neither theoretically or practically affected. --Kubanczyk 06:23, 25 June 2007 (UTC)
[edit] Origin of watercooler raid.jpg?
Does anyone know the copyright status of this image? It's an easy to understand illustration of RAID levels, and might make a great addition to the article, possibly in a redone/computer generated format. Is it accurate too? http://www.edvt.net.nyud.net:8080/Pictures/raid.jpg Family Guy Guy (talk) 23:05, 14 February 2008 (UTC)
[edit] RAID 1 vs 0
The section on RAID 1 seems to imply that there is a data access performance improvement for RAID 1 configuration. I just built a new computer and configured RAID 0 believing that would yield the best performance (albiet with some degradation in reliability). Data access seems to be very fast, but would RAID 1 have been just as fast, with higher reliability? From the data published here, it seems like the best approach would be to add a third drive and move to RAID 5. Is that correct?
Dadman 5 (talk) 21:03, 22 April 2008 (UTC)
- 0 & 1 provide no data integrity.
- 5 does parity so you can at least detect data corruption (no requisite that periodic integrity checks actually be done though), but 5 comes with a computational cost (particularly for software raid).
