DISCLAIMER: I have no professional experience in this field and I've only run home servers with limited storage. Given my limited technical understanding, take whatever I say with a grain of salt.
nortonham What are the pros/cons of using hammer of other filesystems?
Never used HAMMER2, but I've read that it's less resource-hungry than HAMMER and ZFS.
It does most of the relevant things (data/metadata checksummming, snapshots, compression, deduplication, subvolumes / native volume management, native encryption, integrity checks) which ZFS and other new-generation copy-on-write filesystems do as well.
HAMMER2 should have significantly improved write performance compared to its predecessor and does periodic daily bulk pass on the meta-data to free space, thus resolving the essential shortcomings of HAMMER1.
On the other hand, HAMMER1 had transparent journaling capabilities; from the hammer(5) man page:
History
History metadata on the media is written with every sync operation, so
that by default the resolution of a file's history is 30-60 seconds until
the next prune operation. Prior versions of files and directories are
generally accessible by appending `@@' and a transaction id to the name.
The common way of accessing history, however, is by taking snapshots.
I don't see an equivalent feature in HAMMER2.
Compared to HAMMER/HAMMER2, ZFS also brings:
software RAID
self-healing
quotas
resilvering
better cache management (ARC). Also supports using a separate device for caching (L2ARC); the equivalent on DragonFly is swapcache.
native continous TRIM
journaling + SLOG/ZIL. This is similar to the HAMMER1 history retention seen above.
fine grained tuning (adapts better to specific use cases; in particular, you can control how much memory to reserve for ARC, limit i/o throughput rate for write operations).
ZFS has a longer history and a much wider adoption. Compatibility is probably the main reason why probably I'd still opt for ZFS against HAMMER today: the ability to mount an external ZFS-formatted drive on my NetBSD server and export the datasets selectively via NFS, but also the ability to unplug the drive and physically mount it on my Linux workstation, on the desktop PC running Illumos in the basement, and last but not least my sister's macbook.
That said, I always support alternatives. Technologies are 'too niche' only as long as they aren't actively embraced, promoted and used. Years ago nobody was using NPF on NetBSD simply because nobody else used it (a recurrent paradox).