Media Server + Raid 6 - any cache benefit?

[Bigzaj]

I am looking at raid cards for my media server right now. I was looking at the Areca 1680ix-12 with 2-4gb of ram. Now I am reconsidering. Is there any benefit to having more than the standard 512mb of ram for cache for streaming media? (3-4 Blu-Ray at once)

Alternatives now in the fold to be paired with HP SAS expanders are:

ARC-1880-i (2 x SAS port, 512mb ram cache)
ARC-1680-i (2 x SAS port, 512mb ram cache)
LSI SAS 9260-8 (2 x SAS port, 512mb ram cache)

Any thoughts, advice or opinions?

 

[pjkenned]

Not really unless you are also writing to that array while doing those reads. If you are not using the cache as write cache then you are basically hoping the data got cached for reads, which is pretty unlikely if you are streaming 3-4 40GB (120-160GB of files) at once, and then switching to a different set of files. Read cache tends to work well when you have smaller files that fit in cache being accessed repeatedly.

 

[sub.mesa]

It's not just a cache; but it's primarily a buffer. That means, if you write 500MB, it will first enter the DRAM buffer; only later will it be written to the physical disks. Now the trick: the more RAM on the hardware RAID controller, the more you can write without having to touch your disks. So with 2GB RAM on the controller you could be writing a 1.5GB file at memory speeds (though my Areca is limited to 440MB/s buffer write; newer models should be faster).

Otherwise, it would not that important to have more. But if you want to hide your writes the dedicated RAM works well.