ZFS ARC memory requirements

Hello.

I'm planning a machine to be used (almost) only as a backup host.
It will have a "system" SSD-based ZFS pool (possibly 1TB mirror) for OS 
and programs and a large dedicated HDD-based ZRAID1 for backups 
(initially 160-200 TB, but may grow in the future).

I read the rule of thumbs for RAM is 1GB per TB of storage, which would 
possibly mean 192GiB!!!, but I also read this is argued upon.

Considering that:
_ backup data will be organized in "volumes", so I'll have about ~2000 
files totally on the backup pool;
_ i do not plan to use deduplication and I'll probably disable 
compression (since most of the data to be backed up is uncompressible 
and the backup software already tries that anyway);
_ snapshots make very little sense on such a pool (altough I'll use them 
heavily on the "system" pool, but that's small);
_ daily throughput will be probably limited by the network or the 
clients, altough I plan to do monthly "consolidation" of backups and 
periodical disk to tape copies;
_ it's very unlikely data (not metadata) will be read more than once in 
a row, so I don't think it will benefit from caching;

my guess would be that I don't need that much RAM and 64 or even 32GiB 
would be enough.

Opinions?

  bye & Thanks
	av.