Luke Kenneth Casson Leighton wrote:
well. that was nice. the scenario you describe is precisely what i
sort-of had planned, but didn't have the expertise to do so was going
to recommend just two drives and then rsync to the other two.
_however_, given that you've solved exactly what is needed / best /
recommended for when you have 4 external drives (which i do) that's
Great, hope it helps!
ok, i bring in phil now, who i was talking to yesterday about this.
what he said was (and i may get this wrong: it only went in partly) -
something along the lines of "remember to build the drives with
individual mdadm bitmaps enabled". this will save a great deal of
arseing about when re-adding drives which didn't get properly added:
only 1/2 a 1Tb drive will need syncing, not an entire drive
bitmap system he says has hierarchical granularity apparently.
I just added a bitmap, more info. here  &  -- worth a read. You
can add and remove them, sometimes you must remove them ... for
instance, if growing the array.
Quote from  reference:
"In some configurations you may not be able to grow the array until you
have removed the internal bitmap. You can add the bitmap back again
after the array has been grown."
It also seems best to use internal for the bitmap from my reading, so
this is what I did for that.tip:
# mdadm --grow --bitmap=internal /dev/md0
also, he recommended taking at least one of the external drives *out*
of its external box and making it *internal*. the reason for that is
that a) if the drives ever get powered down accidentally (e.g. by
cleaning ladies) you're f****d b) if you move a drive or two
internally, it's possible to prioritise those drives as "reading"
ones, and to make the external ones "write" priority. so, the data
gets read from the internal one, and changes get propagated to all
Yes, that sounds like a good idea(tm) ... worth considering, but right
now I prefer all the RAID drive members external on this particular
machine. The drives and machine are all on a suitable UPS and there is
no cleaning lady to worry about -- my wife won't touch the drives either.
Down the track, I'm sure to move to USB 3.0 and maybe even further down
the track to external PCI Express ... , which looks interesting. And
more so in the future, IBM Racetrack memory  which looks to replace
the "drive" as we know it today.
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