Write performance fluctuates wildly

[Raident]

I have a ~4 GB DVD ISO that I've been copying to the same location on my unRAID server every day for testing purposes.

 

Using reconstruct write mode, on good days I get around 95 MB/s transfer rates and on bad days, I get around 25-35 MB/s. I've also seen a number of transfer rates in between these 2 extremes - sometimes it's ~40 MB/s, other times it might be ~65 MB/s. Also, I'm the only one who uses this server, so there is never more than one connection at any given time when the test is running.

 

What exactly is going on here?