Rob Logan wrote:
> an array of 30 drives in a RaidZ2 configuration with two hot spares
> I don't want to mirror 15 drives to 15 drives
ok, so space over speed... and are willing to toss somewhere between 4
and 15 drives for protection.
raidz splits the (up to 128k) write/read recordsize into each element of
the raidz set.. (ie: all drives must be touched and all must finish
before the block request is complete) so with a 9 disk raid1z set that's
(8 data + 1 parity (8+1)) or 16k per disk for a full 128k write. or for
a smaller 4k block, that a single 512b sector per disk. on a 26+2 raid2z
set that 4k block would still use 8 disks, with the other 18 disks
unneeded but allocated.
It is not so easy to predict. ZFS will coalesce writes. A single transaction
group may have many different writes in it. Also, raidz[12] is dynamic, and
will use what it needs, unlike separate volume managers who do not have any
understanding of the context of the data.
so perhaps three sets of 8+2 would let three blocks be read/written to
at once with a total of 6 disks for protection.
but for twice the speed, six sets of 4+1 would be the same size, (same
number of disks for protection) but isn't quite as safe for its 2x speed.
Yes, need to follow your priorities, there are just too many options otherwise.
-- richard
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