SAS Load Balancing Theory (HP SAS Expander)

[sfcredfox]

Community,

Just double checking with some of the experts here:

I have a DL 380P G8, with a 25SFF backplane in it. It's cabled using two 8087 internal ports, and splits the drives 13 one port, 14 on the other. If you're thinking the math doesn't add up, there's two addition SATA ports on the back of the board that are 26/27 that aren't/wont be used.

Theory:
Since there has to be some over subscribing done if you populated all 25 bays with SSDs, I am thinking about load balancing an SSD pool across both ports.

1) I think it makes sense to split my SSD pool up and populate half in bays 1-13 and the other half in 14-25, at least until I run out of space.

2) My understanding is an Intel DC S3500 800GB maxes around 450-500GB/s (seq best case). This means you tap out a 4 channel 8087 SAS2 6G with about 8 drives?

3) In unlikely case, you could tap out the bandwidth of the cable with 16 SSDs?

4) PCIe 3.0 x8 only does 8GB/s, which still supports around 16 SSDs?

Does that make sense?

My actual workload is random/block/virtualization, so not my biggest concern, just doing some planning.

 

[HoneyBadger]

It's cabled using two 8087 internal ports, and splits the drives 13 one port, 14 on the other. If you're thinking the math doesn't add up, there's two addition SATA ports on the back of the board that are 26/27 that aren't/wont be used.

Wait, really? They split the backplane up? It isn't just a single expander chip and an 8-lane SAS wideport back to the HBA?

For your theoretical questions:

1) If they're actually split in that fashion, then yes - you'd want to distribution the drives ideally between bays 1-13 and 14-25, with my suggested plan being to spread your mirrors across the two for redundancy.

2) 4x6Gbps=24Gbps = 3GB/s, if you're able to push ~512MB per drive then you're maxing out that link at 6 drives.

3) Twelve SSDs in this case, for the same reason as the above.

4) Correct.