Fairly significant factor when building really large systems. If we do the math, there ends up being some relationships between

• disk speed
• targets for ”resilver” time / risk acceptance
• disk size
• failure domain size (how many drives do you have per server)
• network speed

Basically, for a given risk acceptance and total system size there is usually a sweet spot for disk sizes.

Say you want 16TB of usable space, and you want to be able to lose 2 drives from your array (fairly common requirement in small systems), then these are some options:

• 3x16TB triple mirror
• 4x8TB Raid6/RaidZ2
• 6x4TB Raid6/RaidZ2

The more drives you have, the better recovery speed you get and the less usable space you lose to replication. You also get more usable performance with more drives. Additionally, smaller drives are usually cheaper per TB (down to a limit).

This means that 140TB drives become interesting if you are building large storage systems (probably at least a few PB), with low performance requirements (archives), but there we already have tape robots dominating.

The other interesting use case is huge systems, large number of petabytes, up into exabytes. More modern schemes for redundancy and caching mitigate some of the issues described above, but they are usually onlu relevant when building really large systems.

tl;dr: arrays of 6-8 drives at 4-12TB is probably the sweet spot for most data hoarders.