On Thu, Jan 09, 2014 at 05:49:48PM +0100, George Eleftheriou wrote: > Duncan, > > As a silent reader of this list (for almost a year)... > As an anonymous supporter of the BAARF (Battle Against Any RAID > Four/Five/Six/ Z etc...) initiative... > > I can only break my silence and applaud your frequent interventions > referring to N-Way mirroring (searching the list for the string > "n-way" brings up almost exclusively your posts, at least in recent > times). > > Because that's what I' m also eager to see implemented in BTRFS and > somehow felt disappointed that it wasn't given priority over the > parity solutions... > > I currently use ZFS on Linux in a 4-disk RAID10 (performing pretty > good by the way) being stuck with the 3.11 kernel because of DKMS > issues and not being able to share by SMB or NFS because of some bugs. > > I'm really looking forward to the day that typing: > > mkfs.btrfs -d raid10 -m raid10 /dev/sd[abcd] > > will do exactly what is expected to do. A true RAID10 resilient in 2 > disks' failure. Simple and beautiful.
RAID-10 isn't guaranteed to be robust against two devices failing.
Not just the btrfs implementation -- any RAID-10 will die if the wrong
two devices fail. In the simplest case:
A }
B } Mirrored }
}
C } }
D } Mirrored } Striped
}
E } }
F } Mirrored }
If A and B both die, then you're stuffed. (For the four-disk case,
just remove E and F from the diagram).
If you want to talk odds, then that's OK, I'll admit that btrfs
doesn't necessarily do as well there(*) as the scheme above. But
claiming that RAID-10 (with 2-way mirroring) is guaranteed to survive
an arbitrary 2-device failure is incorrect.
Hugo.
(*) Actually, I suspect that with even numbers of equal-sized disks,
it'll do just as well, but I'm not willing to guarantee that behaviour
without hacking up the allocator a bit to add the capability.
> We're almost there...
>
> Best regards to all BTRFS developers/contributors
--
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
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