I work in "sessions", I don't produce content daily. Each session can be anything between 60 to 250 GBs of 30-70MB files + a lot of large tiff files during postproduction. The concern with speed is twofold:
1. for daily backups - Ideally - I'd need that while i'm working so I'd like to minimise any interference with current work. I usually need to deliver work quickly after client approval, so when I'm in the middle of image editing/postproduction I want the backup to be as fast as possible as frequently as possible - which would mean backing up the same large tiff file a few times during the time I spend editing it, or, for that matter, image editing software metadata library, like Lightroom or C1, without slowing down the work. For instance: backing up 1 - 1,5 gig Tiff file is about 10 - 16 seconds at full speed, but I get 30-45 with osx. In the past I had some performance issues with solutions such as time machine so I guess I will have to do more testing to see if that is a real issue or just a psychological effect of knowing that I'm missing out on available hardware performance (btw - switching to PCs only is not an option right now)
2. for archive solution:
I need quick access to about 6 - 12 months worth of content - that can be somehere between 2 and 6 TB. Right now juggling data between ssds and external drives is cumbersome so if I have to do the same with NAS (move data back to local storage) - then what's the point? The main concern is that when I need to re-load and browse through a set of, say 2000, images from 6 months before, every single MB/s counts towards smoother operation and faster work. Hence the question about real life speeds as I suspect in this case It is not feasible to keep all that data on unRaid's ssd cache If that really is about 50 MB/s on average on arrays with parity, then it actually makes more sense to me to invest in more local storage rather than a NAS server.
I guess the problem here is that I'm near that threshold of cost-effectiveness right now. I don't need a large 30+ TB array (where server hardware can be a fraction of the cost of the needed hard drives). I want a fast 4TB array for current backups and/or I'd be happy with 8TB for the archive solution.
Btw - thanks for the input!