Hey there,

 

so this is my first build and serves as storage with parity only, no VMs, apps, dockers. 

I have Gigabit Ethernet providing theoretical read/write speeds of 112 MB/s..

 

Now I started off with two drives, one parity (WD Red 4TB CMR😉), and one storage drive (WD Red 3TB).

All worked well, I could transfer dozens of GBs and always had 112 MB/s writes to the server. (All devices are Gigabit-capable, PC, switch and Unraid)

 

But ever since I added a third drive (Seagate IW 4TB) as a second storage drive, my write speeds drop down from 112MB/s to 40MB/s - max. 60MB/s after pretty much 1GB written to the array. This doesn't change, no matter what drive I write upon or with what allocation method or what SATA port/cable I use.

 

Is this normal?

Yes, both drives worked normally before and I got writes from 190-120 depending on how full the drive is. Same speeds still occur during parity checks.

 

Thanks for any input!

 

 

 

 

PS: Since I mainly write to the array, this is quite important to me. Uploads now take 2-3x longer. And I don't do this professionally, it only stores personal files so I am not in the business for 10 Gbit Networking, because I don't need it. A HDD can't write at 1,12 GB/s anyway.

Wanna share my first theory but abandoned it soon:

 

I am using an M.2 to 5 SATA Port converter.

It uses the JMB585 that connects with PCIe 3 x2, but my NUC Mainboard's M.2 slot gives PCIe 2 x4.

This results in an effective PCIe 2x2 connection, which has a maximum speed of 1 Gbit/s. 

 

This adapter is used by the two data drives, while the parity drive is directly connected via the single SATA port on the mainboard. So I don't see how this is a bandwith issue since only one drive should be written to. Especially since it can reach those 112MB/s for the first Gigabyte...

What you are describing is not atypical.   2 drives (1 x parity + 1x data) is a special case - as soon as you add a second data drive then the overhead of keeping parity in sync becomes more significant (it does not continue to go up as more drives are added).  You may find this section of the online documentation useful in explaining why.   

 

It is likely that enabling "Turbo Write" as described in the link will give you better performance but it will still probably not reach raw drive speeds.  

 

Another alternative is to use a cache drive to that the initial writes go to the cache and then files are later moved to the array in idle periods (i.e. overnight).

This is perfect, I tried with a 6GB file and it stayed at 113MB/s, rock solid!

 

Thank you so much! Turbo write does what I need.