Currently the Time Machine drive is backing up the 2TB (usable) RAID4, 256GB internal SSD in the iMac, and (via Server.app) the 500GB SSD in a MacBook Pro, for a total of 2.75 TB of nominal capacity and probably around 2 TB of actual data. It isn't big enough for a long backup history, but it's plenty sufficient for my needs.How high a percentage of that 4TB Time Machine drive is being used? If you increase the provisioned areas from 2 TB to 3 or 4 TB, is the Time Machine drive now too small? I think the cap for 2.5" hard drives is 5 TB . For the 5-8TB range for a Time Machine drive, switching to 3.5" hard drives might be better. A 3.5" 6TB Iron Wolf drive and 2.5" 5TB Barracuda cost approximately the same.
And while 5TB is the largest 2.5" consumer hard drive currently available, the 4TB I have is already a replacement for a 3.5" drive I was previously using for backup. Cost has nothing to do with it--the 3.5" version was just unnecessarily noisy, even in a fanless enclosure on a styrene pad. The 2.5" drive is much quieter, and I'd rather just stick to smaller, lower-power storage than try to build a soundproof enclosure or something.
Were I to go to a RAID0 + Time Machine option, I would probably get a second 4TB or 5TB hard drive for the fourth bay to use exclusively for backup of the SSD RAID, and delegate the drive I already have to backups of the internal SSDs and/or extra backup or scratch files or something. With the 3xSSD RAID4/5 option I'd be forced to get a 5TB hard drive for the Time Machine drive if and when it got close to full, but that might be several years from now, by which point I could do something else.
In any case, in the next two years the issue isn't backup capacity - I have enough of that - it's how to get the online storage to SSD without costing a fortune.
You may have mis-typed, but three 1TB drives striped as RAID4 is exactly what I have already, and it only provides 2 TB of usable space, not 3 TB, since the third drive is for parity. I only get 3 TB if I fill all four bays with drives and move the backup drive to an external enclosure.I think one of the root causes missed in your list of options is the issue of the drive bay that the Time Machine sucks up. That's part of the capacity problem. If you striped RAID 4 over 3 bays with 1TB drives, you'd have 3 TB of usable space.
Which of course I could do - I could get four 1TB SSDs for 3TB of usable space. That's a 50% capacity increase and only requires 33% overprovisioning rather than 50% to maintain capacity. But I'm quite reluctant to spend nearly $800 for a 50% increase in storage space (and some speed) when I'll be back to having to replace the works in probably 2-4 years.
If I settled for RAID0 + Time Machine, I get double the storage for around the same cost, either have an extra tier of backup or an extra bay available, don't need another external, and don't have to think about refreshing storage for probably 5 years, when I'd be thinking about new drives anyway.
I'd seriously considered this option (added advantage: physical separation of backup and online data), although the unnecessary waste of power of an always-on NAS box annoys me, ones with good Time Machine support tend to be kind of pricey, and I've previously had very poor experience with this configuration.A NAS box as a Time Machine target could be moved away from being highly local, if noise is an issue.
For practical purposes I already have exactly this, except the nearline storage is the 15TB (usable) Thunderbay unit that I don't normally have powered on and has plenty of free space on it. Adding a third tier isn't anywhere worth the mental load of having to pay attention to what's stored where; it's enough as-is to say "Either it's always there or I need to flip a switch to get to it".A workflow change, but another relatively low option would be to use two 2TB SSDs in RAID 1 for critical near-line stuff and to devote one bay to "intermediate candidates for a 3.5-inch box" with a 2TB drive and then a 5TB Time Machine drive that backs up both of those two volumes.