I don't know how best to report this. Its a pure ZFS send issue that wasnt the case on 11.3, so logs show nothing, and it doesnt cause a crash or exception either, so theres no system dumps.
I have a pool with this exact structure:
(~ 16000 snaps total in pool)
I've replicated this pool many times to my backup NAS, at times when the snapshot count was around 80000. But it wont zfs send on 12-BETA, and the issue looks like some extremely low limit (~ 1000?) on the number of snapshots it'll send.
If I try to zfs send anything more than about 1000 snaps, I get an error at the "send" side.
total estimated size is nnnG cannot hold: operation not supported by zfs kernel module cannot send 'Main_pool/User_files': operation not supported by zfs kernel module
The issue isn't about the size of the send - it will send -R the first 1000 snaps from 2017 to July 2018, which total 35 TB. But if I try to send the rest of the pool after it, using send -R -I from that snap to the current snap, or to the 2018-12 or even the 2018-10 dated snaps, I get the above error. If I reduce the number of snaps in the stream to around 1000, by only sending -I from the end of the 35TB stream to 2018-09-01, which is around 1000 snaps, it'll send that happily, and let me send a followup stream, as long as that doesnt have more than about 1000 snaps in it.
As the error doesnt result in a log entry, I'm not sure what other info would help. dtrace maybe, if I was told what to run?