I do not own the SAS 9305-16i, but according to ServeTheHome, this SAS controller should work with FreeNAS.
We have our top picks for getting fast and reliable FreeNAS HBAs (host bus adapters) for SAS and SATA, using proven options for FreeNAS and ZFS
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I cannot tell you if the card is already flash in IT mode, you have to see with the vendor. But you do not have to worry: flashing in IT mode is easy because TrueNAS comes with the Broadcom utility
sas3ircu
.
Another option to connect a lot of disks is using a
SAS expander, like in SuperMicro servers. In this case you need only one SFF8643 connector per expander.
Most servers have only 2 SAS expanders, one in the front, and one in back, so a LSI 9300-8i with 2x SFF8643 is enough to connect all the disks.