On Fri, 2013-07-05 at 09:36 -0400, Phil Turmel wrote:

You picked "redundancy".  Leaves only only one axis to consider: speed
vs. capacity.

Well thinking about that "raid6check" tool you told me over in the other
thread,...
which AFAIU does what I was talking about, namely telling me which block
is the correct one if I have bad blocks (and the disk itself can't tell)
and not whole drive failures,.. where at least a two-block copy RAID10
would not be able to...
...then I think RAID6 is THE solution for me, given resilience has the
highest priority, as RAID10 with c/f/o=3 cannot do that.

Precisely.  This is "raid10,near3".  You can look up the "offset" and
"far" variants.

Actually I had written some ASCII art visualisations for the Debian
mdadm FAQ (and perhaps also the md(4) manpage) yesterday... I just wait
for some minor answers from Neil to publish them... so I already had a
look at these.
But AFAIU, they make absolutely no difference for resilience... and
while far/offset improve performance in some use cases,... they make it
worse in others.

quoted

And that gives me again, any 2 disks... but so much better performance?

Dramatically.

I guess I'll do some benchmarking.. ;)

At four drives, no.  Any two.  With five, there are some combinations of
three missing drives that'll still run.

Was thinking about that as well... but as you said it's a "it might
survive"... not a "it will survive", right?!

quoted

Any ideas about the layout? i.e. left-symmetric-6, right-symmetric-6,
left-asymmetric-6, right-asymmetric-6, and parity-first-6 ?

Certainly not any of the "-6" suffixes.

Ah.... I just see that the RAID5 layouts are also for RAID6 ^^

Those isolate Q on the last
disk, hurting streaming performance, and setting up the possibility of
uneven performance when degraded.  The default left-symmetric gives the
best chunk distribution for general use.

I see... I was just searching for but haven't found anything really
good... actually nothing at all ^^
Does anyone know some place where the layouts for RAID5/6 are
(thoroughly) explained?


Cheers & thanks,
Chris.