Question about unRAID Cache

[bcp]

So I've recently acquired all the pieces for a home NAS i've wanted to build for some time now. Going to do the build this coming weekend. I do have a couple questions and any comments/tips would be greatly appreciated. I I think I've decided on using unRAID, or at least going with the trial for the time being (yes i know its not free). The reasoning behind this is the easy expandability by simply adding a drive, and the ability to add drives that are not identically sizes etc.  If this is a false assumption then please correct me. 

 

Amongst the parts I'm using I have 3 x 3tb WD Red drives. One for a parity drive and the other two for storage. My main use for this server will be 3fold. 1) As a plex media server, streaming to several devices around the home and to select family and friends around the county. 2) As a torrent downloading HUB. This lets me take the current 3tb WD red out of my machine and if I need to do maintenance/repairs/upgrades or if im just on the road I can still download tv shows etc to the server to watch. 3) As a backup for my PC and laptop for most of my school/research documents. I have an external HD that I also use, but "3-2-1 !".  There may be other uses after time, but as of now these are my main three for this build. My main question is whether or not I would see a large performance increase by adding an SSD cache to this unRAID array? I semi-understand how it works. File transfers go tot he cache initially, and it the dumps to the HDD's at set times through the day. Basically this limits my maximum file transfer size to the size of the SSD.  Would I see a noticable benefit in the 3 uses I listed by adding an SSD into this array? If so how large should I go. My markedly uneducated feeling was a 240-256gb ssd would be appropriate for my use, but thinking about it I can't think of a single file transfer I would make that would be over even 120gb. 

 

TL;DR - Building a NAS with 3x3tb WD Reds. Do I need an SSD as a cache, and if so how big?

 

Thanks for hte help in advance, and I hope my understanding of how this works isn't too messed up!

[Mayaa]

What kind of network speeds are we looking at? the 3 WD Reds should easily saturate a 1Gbit connection, speed wise.

If you need more speed than that, which i doubt, then go ahead with the cache.

[pat-e]

The cache is not generally for the speed bandwidth wise, it is more for the possible IOPs (like comparing copy 1 big file is much faster as many small files). The SSD can speed up this transfer of small files (the read is less the problem than the write).