At least TrueNAS supports assigning SSDs for metadata and especially in case deduplication is used for ZFS:
High quality mirrored SSDs configured as a special vdev for the DDT (and usually all metadata) are strongly recommended for deduplication unless the entire pool is built with high quality SSDs.
https://www.truenas.com/docs/references/zfsdeduplication/#disks
A pool must always have one normal (non-dedup/special) VDEV before other devices can be assigned to the special class.
The metadata special VDEV is critical for pool operation and data integrity.
Drives added to a metadata VDEV cannot be removed from the pool.
https://www.truenas.com/docs/core/coretutorials/storage/pools/fusionpool/
I'm considering setting up some TrueNAS with external USB HDD backup of internal data. Some of the internal data is known to contain duplicates, but not too sure how much. Though, the internal storage of the device is easier larger than the USB disks used, so what doesn't matter too much internally might matter a lot externally.
Therefore I considered using ZFS with deduplication at the USB disk only. I know that deduplication needs a lot of RAM or some special SSD devices like quoted above and from my understanding, using SSDs would be a somewhat easy way to not "waste" too much RAM on deduplication only. Especially because I don't need max. performance when backing up to USB most likely, it just needs to succeed within some hours.
So, is it possible to use SSDs being part of the internal pool hold the deduplication tables for the external USB-pool?
Doesn't read that way to me. Instead, it reads to me like if deduplication is used with USB, than RAM needs to be available to hold the deduplication tables of that pool.
