SAS and Sata in one pool but separate HBAs and Vdevs

[AVB]

I have an 846TQ chassis with two Dell H310 HBAs flashed to IT mode running two 8 drive ZFS2 Vdevs. I want to upgrade one of the Vdevs with SAS drives. I know I can't mix the drives on one HBA but they will be on completely different HBAs, just the same pool. Possible? Pros - Cons?

Appreciate any and all answers.

 

[sretalla]

DIfferent speeds in one pool can produce performance confusion since it won't be consistent or predictable, but otherwise, I see no barrier to it.

 

[AVB]

DIfferent speeds in one pool can produce performance confusion since it won't be consistent or predictable, but otherwise, I see no barrier to it.

The drives are supposedly the same 6g speed just the capacities are different. The SAS drives are 6TB the Sata drives are 4TB.

 

[Chris Moore]

Unless you are saving a lot of money on SAS drives instead of SATA drives, I would say that it is better to stay with SATA. The diagnostic data provided by a SATA drive is usually more meaningful / useful. SAS drives are only better for some very particular things, but if you are getting a really good deal on them, buy extra for cold spares.

I know I can't mix the drives on one HBA

Why not? There have been people post in the forum with a single pool that is attached to an expander backplane where one controller is running all the drives and half are SATA and the other half are SAS.
Personally, I have not tried it, but I don't know where the idea comes from that it won't work.

 

[PhiloEpisteme]

The drives are supposedly the same 6g speed just the capacities are different. The SAS drives are 6TB the Sata drives are 4TB.

I imagine the HDDs are not actually 6g speed. They are probably closer to 1g per drive. Often drives will describe the interface they support which is often 6g for SATA 3 or SAS2.

 

[Chris Moore]

I imagine the HDDs are not actually 6g speed. They are probably closer to 1g per drive.

If you dig into the specs, the fastest drives (SAS or SATA) are usually around 250 MB/s. It depends on the mechanicals of the drive. The one I linked to below isn't quite that fast, but the table line to look at is called "Max. Sustained Transfer Rate", so it may burst faster than that and it will definitely go slower that that. On average, I see 110 MB/s out of the drives in servers at work. So, the 6GB rating on the controller is more about marketing and 12GB SAS controllers are totally not-needed unless you are using more than around 80 of the fastest mechanical drives around.

https://www.seagate.com/www-content/datasheets/pdfs/3-5-barracudaDS1900-11-1806US-en_US.pdf

PS. Older drives, and lower capacity drives, tend to be slower where the more modern drives tend to be faster. Part of it is down to the mechanical device being better, but it is largely due to greater density of the data on disk which means the read/write head doesn't have to swing around as much.

Also, keep in mind that sequential access is going to be fast where any access that requires seek operations are going to be much slower because of the seek time.