I originally added this as as reply to [6.7.2] DOCKER IMAGE HUGE AMOUNT OF UNNECESSARY WRITES ON CACHE
Didn't notice it was raised against 6.7.2, not 6.8.0. Sorry]
For what it's worth:-
Is this perhaps an issue with sparse files ?
I'm having a somewhat similar problem when writing a torrent of an ISO to a share on a mirrored cache pool.
(2 x Crucial MX500 SSDs, BTRFS, Raid 1)
6.7.2 was fine, on 6.8.0 there's huge write amplification whenever the torrent client tries to write out a chunk.
e.g. Torrent client creates a 3.5GB sparse file, then starts downloading chunks to it's internal RAM cache. When a chunk (4MB) part is completely received it writes it to disk into the pre-allocated ISO sparse file.
However - instead of the expected 4MB disk write it seems to be rewriting the *entire* 3.5GB file every time it sends a new chunk to the disk file.
This leads to the SSDs writing continuously for hours, for what *should* take less than a second (and also getting very hot).
Other types of disk write activity (copying, moving, file creation etc) behave normally, at expected speeds and levels of SSD activity. i.e. Copying a regular 3.5GB file to the share takes < 10 seconds.
Also - I reverted from 6.8.0 to 6.7.2 and the problem disappeared, so it's not related to a bad Unraid cache pool config, or anything in the, unchanged, torrent client config.
I found this bug report before I raised one of my own and I wonder if it was the same root cause as (I believe) the Docker IMG files are also created as sparse files.
If I'm way off base, and my issue is unrelated issue please let me know and I'll raise mine as a separate bug report.