On 2016-08-15 09:39, Martin wrote:
That really is the case, there's currently no way to do this with BTRFS.
You have to keep in mind that the raid5/6 code only went into the mainline
kernel a few versions ago, and it's still pretty immature as far as kernel
code goes. I don't know when (if ever) such a feature might get put in, but
it's definitely something to add to the list of things that would be nice to
have.
For the moment, the only option to achieve something like this is to set up
a bunch of separate 8 device filesystems, but I would be willing to bet that
the way you have it configured right now is closer to what most people would
be doing in a regular deployment, and therefore is probably more valuable
for testing.
I see.
Right now on our +500TB zfs filesystems we used raid6 with a 6 disk
vdev, which is often in the zfs world, and for btrfs I would be the
same when stable/possible.
A while back there was talk of implementing a system where you could
specify any arbitrary number of replicas, stripes or parity (for
example, if you had 16 devices, you could tell it to do two copies with
double parity using full width stripes), and in theory, it would be
possible there (parity level of 2 with a stripe width of 6 or 8
depending on how it's implemented), but I don't think it's likely that
that functionality will exist any time soon. Implementing such a system
would pretty much require re-writing most of the allocation code (which
probably would be a good idea for other reasons now too), and that's not
likely to happen given the amount of coding that went into the raid5/6
support.
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