RAID for Four Disks
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This unique question comes up so often that it warrants a reference document just for it, even though standard RAID information would cover this if examined thoroughly. So here is the basic question:
I have a four drive disk array of traditional hard drives, what RAID level should I use?
- Where "traditional" means Winchester drives - spinning mechanical drives.
Quick Answer: RAID 10, every time.
Long Answer:
In a four drive array there are five physically possible configurations of drives: RAID 0, RAID 5, RAID 6, RAID 10 and two independent RAID 1 arrays.
RAID 0 provides no drive failure protection and no URE protection. Unless this is a cache system where no data on the drives matters this isn't an option at all. So we rule RAID 0 out immediately.
RAID 5 was deprecated for spinning drives circa 2009 and, like RAID 0, is simply not a rational option. It would be too costly or too risky (depending on drive factors) to consider so no need to consider it.
Dual RAID 1 is safe but is pointless array splitting and would result in wasted performance and capacity for no reason. It is technically safer than RAID 10, but only in the most trivial way so that in any case where more reliability than RAID 10 was needed, this would not be enough to be considered. So that cannot be a factor. But that it is slower and less useful in capacity should rule it out. Also more cumbersome to manage.
RAID 6 is physically possible but not considered an option until we reach five drives in our array. RAID 6 is safe enough to use, but creates a lot of overhead. In normal usage we only use RAID 6 over RAID 10 when we want the additional capacity potential of RAID 6 and are willing to sacrifice the speed and reliability of RAID 10 to get it.
RAID 10, with only four drives, has no capacity loss compared to RAID 6, their capacity utilization is identical. So the one benefit of RAID 6 does not exist in this case. So RAID 10 is faster and safer, with identical capacity. So there is no need to consider RAID 6 as RAID 10 is better in every scenario.
Because of the factors at play here, there are truly no niche scenarios where other RAID levels could be considered. No matter what the application of the storage is, only RAID 10 makes sense.
Reference:
http://www.smbitjournal.com/2012/11/choosing-a-raid-level-by-drive-count/
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This is a "standard pattern" that is worth memorizing. Outside of the RAID 0 cases for truly worthless temporary storage (in which case we assume that this question would not even arise) there is no need to understand the business case. RAID 10 is always the answer. At five drives, that of course changes. This is unique to the four drive scenario.
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Of course the question comes up: what about if these are SSDs. And that changes everything. There are more options and RAID 5, 6 and 10 are all options but RAID 5 would be the most common, RAID 10 would be okay and we'd likely avoid RAID 6 for performance reasons. But technically, in that setup with good SSDs, RAID 6 would likely beat RAID 10 in reliability under normal circumstances.
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Nice post, but as you mentioned, it's a seemingly unnecessary one because of the obviousness of the situation.
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@iroal said in RAID for Four Disks:
Try to explain this to my boss.
For him Raid 10 does not exist only Raid 5 or 6..
Is he simply unaware that RAID 10 exists?
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@Dashrender said in RAID for Four Disks:
Nice post, but as you mentioned, it's a seemingly unnecessary one because of the obviousness of the situation.
YOu would think, yet it might be the most common RAID situation that I see asked about.
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@Dashrender said in RAID for Four Disks:
@iroal said in RAID for Four Disks:
Try to explain this to my boss.
For him Raid 10 does not exist only Raid 5 or 6..
Is he simply unaware that RAID 10 exists?
He never had a problem with Raid 5/6 so why to change.
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@iroal said in RAID for Four Disks:
@Dashrender said in RAID for Four Disks:
@iroal said in RAID for Four Disks:
Try to explain this to my boss.
For him Raid 10 does not exist only Raid 5 or 6..
Is he simply unaware that RAID 10 exists?
He never had a problem with Raid 5/6 so why to change.
I take it he didn't have large HDD RAID 5 arrays? then see a resilver fail due to URE or a second drive failure?
But like you boss, I've also never had an issue with RAID 5, and I've never used a RAID 6. Until I joined SW, I didn't realize the need to move away from RAID 5 due to mathematics for URE chances. Understanding this shows that the reasons for using RAID 5, single drive failure survival, is not longer valid. So we must move to RAID 6 or 7 or 10 to gain that drive failure situation at or better than we had at RAID 5.
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@iroal said in RAID for Four Disks:
@Dashrender said in RAID for Four Disks:
@iroal said in RAID for Four Disks:
Try to explain this to my boss.
For him Raid 10 does not exist only Raid 5 or 6..
Is he simply unaware that RAID 10 exists?
He never had a problem with Raid 5/6 so why to change.
I keep trying to reach for things he might never have had a problem with, but upgrade/changed anyhow, but for each example I think of failures/reasons that the upgrade happened. But the problem with that is that from what you've said the boss himself never experienced a problem. This is just one of those things you need to understand the math and the reason for doing what you were doing before. If he doesn't agree with those things, well, then there's a total lack of understanding either by you or by him, and until you're on the same page, there won't be an agreement.
Question - what was his reasoning for doing RAID 5/6 in the first place? and why did he start using RAID 6 instead of 5? what a waste of a drive, right?
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@iroal said in RAID for Four Disks:
@Dashrender said in RAID for Four Disks:
@iroal said in RAID for Four Disks:
Try to explain this to my boss.
For him Raid 10 does not exist only Raid 5 or 6..
Is he simply unaware that RAID 10 exists?
He never had a problem with Raid 5/6 so why to change.
Redefine "problem" for him. I'd say he's had lots of problems. Ask if "risking the business for no reason" is a problem
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Is most of the confusion about RAID happens a lot more when using it has VM storage?
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@black3dynamite said in RAID for Four Disks:
Is most of the confusion about RAID happens a lot more when using it has VM storage?
Why would that matter? If anything, from the VM point of view life just gets super easy - don't care about storage, it's not it's job to worry about anymore. The underlying host manages that.
But the underlying principles of what storage to use still exist. As I'm trying to keep more forward in my thinking there are many options, and you need to pick the correct one.
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@black3dynamite said in RAID for Four Disks:
Is most of the confusion about RAID happens a lot more when using it has VM storage?
That appears to be the case. Virtualization and RAID seem to just universally confuse, I suppose.
