I came across Unraid about a week ago and I'm going to be doing a build to test it out.  I have 4 4TB drives on the way and I'm using a used computer that has 6 SATA connections on the motherboard.  The plan is to build it with 3 data drives and one parity drive.  I don't have a SSD drive in the build yet but I do have a 1 TB drive sitting around that I could use as a cache drive.  Is there any real advantage in using another SATA drive as a cache?  I know that SSD drives are usually used because of their higher speed, but with just a normal SATA drive will I see any speed increase?  Or maybe it is just not worth it.

 

Build:

HP Envy 700-406

12 gigs memory

AMD A10 7700k processor

used.  $100

4 Ironwolf 4 TB hard drives (new)

 

Thanks

 

 

 

Edited July 27, 20196 yr by ddr
grammer

Yes, even a normal HDD drive provides a measurable performance benefit.

 

the thing to realise is that ‘writes’ to the parity protected array are more complicated than you might think!   Basically a single ‘write’ involves reading the target sector from the target drive and the corresponding sector on the parity drive; calculating the updated contents of these sectors; waiting for the drives to complete a disk rotation; writing the updated sector to the target drive and the parity drive.    This is why writing to a parity protected array is always significantly slower than raw disk performance suggests should be the case.

 

when you have a cache disk then for writing any new files to a User Share configured to use the cache then the write occurs at the normal speed for the cache drive.   The downside is at that moment the data is not yet protected.   Later when the mover runs the data is moved to the main array so that it becomes protected.   Mover is normally scheduled to run overnight when the end-user will not notice the slower write speed to the array.

Thank you itimpi for taking the time to answer that.

3 hours ago, ddr said:

Or maybe it is just not worth it.

Another good reason for having a cache drive is for appdata and/or VM storage.  In addition to the write caching benefits, it provides a dedicated location for storing docker containers and their associated configuration files without creating a share on the array.  Personally, I use an SSD as my cache disk (I do not cache any share writes) solely to store appdata, downloads and handbrake converted files.  I also have another SSD as an unassigned device (not part of the parity-protected array) that I use for storing VM related files.