The issue with this, in my experience, is that at some point that pro (exactly, and literally, only one copy of a specific bit of data despite many apparent copies) can become a con if there is some data corruption somewhere.
Sometimes it can be a similar issue in some edge cases performance wise, but usually caching can address those problems.
Efficiency being the enemy of reliability, sometimes.
Redundant copies on a single volume are a waste of resources. Spend less on size, spend more on an extra parity drive, or another backup of your most important files. That way you get more safety per gigabyte.
Notably, having to duplicate all data x2 (or more) is more of a waste than having 2 copies of a few files - if full drive failure is not the expected failure mode, and not all files should be protected this heavily.
It’s why metadata gets duplicated in ZFS the way it does on all volumes.
Having seen this play out a bunch of times, it isn’t an uncommon need either.
Well I didn't suggest that. I said important files only for the extra backup, and I was talking about reallocating resources not getting new ones.
The simplest version is the scenario where turning on dedup means you need one less drive of space. Convert that drive to parity and you'll be better off. Split that drive from the pool and use it to backup the most important files and you'll be better off.
If you can't save much space with dedup then don't bother.
> There was an implication in your statement that volume level was the level of granularity, yeah?
There was an implication that the volume level was the level of granularity for adding parity.
But that was not the implication for "another backup of your most important files".
> I’m noting that during on volume wide dedup can have the con that you can’t choose (but it looks like you can!) to manually duplicate data.
You can't choose just by copying files around, but it's pretty easy to set copies=2 on specific directories. And I'd say that's generally a better option, because it keeps your copies up to date at all times. Just make sure snapshots are happening, and files in there will be very safe.
Manual duplication is the worst kind of duplication, so while it's good to warn people that it won't work with dedup on, actually losing the ability is not a big deal when you look at the variety of alternatives. It only tips the balance in situations where dedup is near-useless to start with.
Sometimes it can be a similar issue in some edge cases performance wise, but usually caching can address those problems.
Efficiency being the enemy of reliability, sometimes.