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Having 10 disks, I want to use one quarter of each for a big raid 0 array and the remaining 3/4 of each for a raid 1 array having a backup of the raid 0 array. I can't do a raid 1 or both array, that would slow down the raid 0 speed. I need 99.9% reads, mostly concurrent. Raid 1 on 2 disks doesn't seem to double the read speed to me. This is why I want a raid 0.

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Sorry for hitting your brain. It wasn't my goal. Also sorry for my English. Point is that in raid 10 for 10 disks the read speed is 5*speed of the single drive, while in raid 0 it's 10*single drive speed. I was wondering if there was a way to combine 2 array, reading data only from the raid 0, while keeping the raid 1 for redundancy. I'm again sorry for how bad I explain myself, really. – cedivad 10 mins ago 

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closed as not a real question by ewwhite, JamesRyan, MadHatter, Ward, Tom O'Connor Nov 14 '11 at 23:32

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.If this question can be reworded to fit the rules in the help center, please edit the question.

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You forgot to ask a question. –  David Schwartz Nov 14 '11 at 9:52
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What is wrong with raid 10? Given your read ratio there will be no drop in performance from raid 0 –  Mark Lawrence Nov 14 '11 at 9:56
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Question or no question, why are you even considering RAID 0? I'm not the first to say this and I won't be the last but RAID IS NOT BACKUP (especially if it's in the same machine). –  tombull89 Nov 14 '11 at 9:57
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This site is for Professional Sysadmins, and Pros never use R0 for anything they even remotely care about. –  Chopper3 Nov 14 '11 at 9:58
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@Chopper3 - thinking your comment is a bit unfair - but the question is definitely back to front - choice of raid type should depend on requirements not vice versa. –  symcbean Nov 14 '11 at 10:01

1 Answer 1

This is why RAID10 was invented. Double the speed, double the redundancy.


Ok, after seeing your update, consider these RAID-0 figures for load time for let's say a 10Kb file (totally ficticious, but good for illustrating a point. They're totally inaccurate for real-life scenarios because they don't take into account RAID controller overheads, caches, etc):

1 Disk:  10.0Ms
4 Disks:  2.5Ms
8 Disks:  1.25Ms
16 Disks: 0.66Ms

Or for a basic RAID10:

1 Disk:  10.0Ms
4 Disks:  5.0Ms
8 Disks:  2.5Ms
16 Disks: 1.25Ms

You're losing a grand total of about 1.25Ms (in this ficticious example). In real life, these numbers are smaller and even more trivial.

The more disks you add, the lower your "return" of each additional disk will be. By the time you're up to 10 or 16 disks, each additional pair of disks only adds the tiniest amount of additional speed.

Also, as mentioned in the comments below, a decent RAID controller will use all the avaliable disks for reading, so over RAID-0 you lose on write speed, but not read.

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Why do you think that? –  MadHatter Nov 14 '11 at 11:08
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This is entirely implementation-dependent. A bad RAID1/10 driver won't load-balance the reads across all the spindles, but a good one will. If you've bought a cheap RAID card, or are using poor software RAID, buy more discs. If you want to get good value out of your discs, get a decent card/implementation. Note also that read workload is important: even poor versions may well load-balance reads when the workload is multiple small random reads; balancing sequential reads is more difficult. –  MadHatter Nov 14 '11 at 11:23
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My workload is lots of 4kb files. I'm using a megaraid 9260, can't change it. :( –  cedivad Nov 14 '11 at 11:29
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So am I right in thinking that your question is: I'm using a MegaRAID 9260 card and 10 discs, I only get half the read performance using RAID10 vs RAID0, how can I get the redundancy of RAID10 with the read performance of RAID0 without changing the card? Because if so, the answer's simple: you can't. –  MadHatter Nov 14 '11 at 11:50
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There are solutions that use SSDs for caching and spiny disks as a tier 2 storage; they aren't even that expensive (relatively speaking). –  Chris S Nov 14 '11 at 20:00

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