do you have more info on this? I'd love to read a blog post about it. Are there any performance implications to doing ( or not doing ) this?
https://rsync.net/resources/notes/
... we've not disussed this item yet but I am finalizing the most recent "notes" and will include it.
If you don't have busy filesystems filling beyond ~90% (used to be 80%...) then I wouldn't ever worry about this.
OTOH, if you have very busy filesystems with lots of different consumers and hundreds of millions (or billions) of inodes then a recent dataset might be scattered all over your zpool. There are absolutely performance ramifications to this.
So you expand that zpool (you have to expand it for this to work) by adding a vdev, and then you 'zfs send' that dataset onto the same zpool and it will be laid down nice and orderly onto the new free space that was added to the zpool.
That dataset will then be more performant. This is analogous to "defrag" as we think of it but, again, in a duct-taped-engineering kind of way.