If k=8,m=3 is too slow on HDDs, so you need replica 3
and SSD DB/WAL,
vs EC 8,3 on SSD, then that's (1/3) / (8/11) = 0.45 multiplier on the
SSD space required vs HDDs.
That brings it from 6x to 2.7x. Then you have the benefit of not
needing separate SSDs for DB/WAL both in hardware cost and complexity.
SSDs will still be more expensive; but perhaps justifiable given the
performance, rebuild times, etc.
If you only need cold-storage, then EC 8,3 on HDDs will be cheap. But
is that fast enough?
Ok, I understand.
We have a "hot" fraction of our dataset - and 10GB cache on all 113 HDD
~1TB effective read-cache - and then writes hitting the battery-backed
write-cache - this can overspill and when hitting "cold" data performance
varies. But the read/write amplification of EC is still un-manageable in
pratice on HDD with an active dataset.
--
Jesper