ZFS pool fragmentation - plan to reduce?

[Arwen]

This is a general ZFS pool question, not specific to FreeNAS.

One of my root pools is badly fragmented. It's a 20GB mirrored pool with 72% fragmentation right
now, (but only 52% used).

Would a ZFS send & receive from existing root pool to a newly created pool reduce fragmentation?

Basically I can break off a mirror and use it for the destination. (And yes, I have excellent backups.)

Back story. This is my minture media server running Linux with ZFS. Originally built without space
on the 2TB spinning rust for a mirror of "/boot" or "swap". Thus, my backup, repartition and restore
of the Media on it, which reduced the fragmentation to 1% or so, down from 11%. (And now I have
room to mirror "/boot" and "swap".)

 

[Ericloewe]

Would a ZFS send & receive from existing root pool to a newly created pool reduce fragmentation?

Most likely. I don't think there are any mechanisms to guarantee it, but an empty pool recving stuff will naturally try to fragment free space as little as possible.

 

[rs225]

It will(for a while). But if you actually need to do this for performance reasons, you should actually move that to SSD.

That's what I did next.

 

[Arwen]

Well, it turns out my first few hours of re-loading the Media gave false indication of fragmentation.
After 2/3s of my Media has been restored, the fragmentation is now at 33%. That's 3 times higher
than before, which was 11%.

As for performance reasons, no, I am not doing it for performance reasons. I may just ignore the
root pool for a while, since it does not seem to cause any specific issues.

PS - My miniture media server has 1 x mSATA slot, (1TB installed), and 1 x 2.5" wide, 9mm high
disk slot, (2TB HDD installed). So I am mirroring from SSD to spinning rust. Not ideal, but it's just
a media server. I'd rather have the reliability of mirroring, than worry about bad blocks.

 

[rs225]

I think fragmentation(the value) doesn't mean what we think, because I also see numbers that make no sense. The use pattern and actual performance is what you have to watch.

 

[Arwen]

I think fragmentation(the value) doesn't mean what we think, because I also see numbers that make no sense. The use pattern and actual performance is what you have to watch.

I am starting to agree.