I'm trying to understand how this works. I have read the wiki but seek a little more in depth understanding.I know it is faster because the data gets written to the cache drive and then parity calculated later in the night etc.

 

Does it write some data to the data drive and some on the cache drive to achieve this speed (twice as fast because the data is split in half)? Or does it write all to the cache drive, thereby bypassing the parity calculation for later (hence the speed)?

Writes all the data directly to the cache disk. Then at some time later, it moves the data onto the parity protected array.

 

And if I understand correctly, whatever is on the cache drive is not protected until it gets written to the data drives.

And if I understand correctly, whatever is on the cache drive is not protected until it gets written to the data drives.

 

That is correct.

Yes, that is correct. Unless you have an exotic setup with a hardware RAID-1 cache drive the data on the cache drive is not protected until it's moved into the unRAID Array. In particular, this sort of setup: http://lime-technology.com/forum/index.php?topic=7150.0

 

 

 

 

Therefore beware of trying to run a copy of a volume of data which exceeds the capacity of the cache drive!  I tried to do this with some raw videos, not thinking about how the cache drive would respond.  The results were not good. 

 

If you need to copy large volumes of data and use a cache drive, then make sure it has sufficient capacity to perform the copy.