On 2016-08-15 10:32, Anand Jain wrote:
On 08/15/2016 10:10 PM, Austin S. Hemmelgarn wrote:
On 2016-08-15 10:08, Anand Jain wrote:
IMHO it's better to warn user about 2 devices RAID5 or 3 devices
RAID6.
Any comment is welcomed.
Based on looking at the code, we do in fact support 2/3 devices for
raid5/6 respectively.
Personally, I agree that we should warn when trying to do this, but I
absolutely don't think we should stop it from happening.
How does 2 disks RAID5 work ?
One disk is your data, the other is your parity.
In essence, it works
like a really computationally expensive version of RAID1 with 2 disks,
which is why it's considered a degenerate configuration.
How do you generate parity with only one data ?
You treat the data as a stripe of width 1. That's really all there is
to it, it's just the same as using 3 or 4 or 5 disks, just with a
smaller stripe size.
In other systems, 4 is the minimum disk count for RAID5. I'm not sure
why they usually disallow 3 disks (it's perfectly legitimate usage, it's
just almost never seen in practice (largely because nothing supports it
and erasure coding only makes sense from an economic perspective when
dealing with lots of data)), but they disallow 2 because it gives no
benefit over RAID1 with 2 copies and gives worse performance, not
because the math doesn't work with 2 disks.
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