SAS3 SSDs on SAS2 card and HBA

[FiberGeek]

I have some extra WD SS530 SSDs left over from our FreeNAS upgrade project, and i'd like to use a couple in our VM hosts. However these host only have SAS2 backplanes and HBAs.

Would this be an issue at all? We only plan to use 2 drives per host in a zfs mirror configuration. (Proxmox)

Here is what's listed for the SSD performance:

• Internal Data Rate: 2150 MBps
• Internal Data Rate (Write): 2120 MBps
• 4KB Random Read: 440000 IOPS
• 4KB Random Write: 240000 IOPS

 

[gea]

No problem.
You will only be limited by 6G specs (mainly half the sequential transfer rate)

 

[FiberGeek]

I guess what I'm really trying to figure out is if those drives will be actually be bottlenecked. I'm not filling every bay on the server, just 2 of them.

 

[gea]

Similar to a car where you buy wheels that allow 250 km/h when your car is limited to 150 km/h

 

[FiberGeek]

That analogy doesn't really work well here.

These drives are able to do 2.1G each. So in theory they should be able to do 4.2G (max) together thus under the 6G limit. I was just wondering since it's on a SAS2 HBA, it for some reason won't be able to get it's full speed. I figured if I was to add more ssds past 3, then it would be a problem and bottleneck to 6G. ALSO, the backplane I have connects with 2 SAS cables, so I can probably make sure each drive is on a seperate 6G lane.

 

[ske4za]

You're confusing megabits and megabytes there. SAS2 is 6Gb/s (lower-case b, gigabits), which is 750MB/s (capital B, megabytes). This is not accounting for overhead - max throughput is probably 500-550MB/s.

Further complicating things are those specs in the datasheet for SS530 drives are actually in MiB, which are mebibytes. 1 MiB = 8.388608 Mb (megabits), and 1.048576 MB (megabytes). In the footer of the datasheet:

1MiB=1,048,576 bytes or 2^20, 1KiB= 1,024 bytes or 2^10.

So:
2150 MiB/s = 2254 MB/s = 18 Gb/s

SAS3 is 12Gb/s, or 1500MB/s not accounting for overhead. So you'd never see the full potential of those drives anyway, and even less in a SAS2 configuration with regards to max throughput.

 

[FiberGeek]

You're confusing megabits and megabytes there. SAS2 is 6Gb/s (lower-case b, gigabits), which is 750MB/s (capital B, megabytes). This is not accounting for overhead - max throughput is probably 500-550MB/s.

Further complicating things are those specs in the datasheet for SS530 drives are actually in MiB, which are mebibytes. 1 MiB = 8.388608 Mb (megabits), and 1.048576 MB (megabytes). In the footer of the datasheet:

So:
2150 MiB/s = 2254 MB/s = 18 Gb/s

SAS3 is 12Gb/s, or 1500MB/s not accounting for overhead. So you'd never see the full potential of those drives anyway, and even less in a SAS2 configuration with regards to max throughput.

Okay, so would the IOPS take a hit a too? We are not really doing big data transfers, but we do have a crap load of VMs that have DBs in them.

Here is the exact spec sheet of the drives I got.

EDIT: They list the speeds as 2150 MBps not MiB/s

 

[ske4za]

I was looking at the datasheet directly from WD: https://documents.westerndigital.co...-sas-series/data-sheet-ultrastar-dc-ss530.pdf

I'm sure IOPS would, but I wouldn't be able to tell you by how much. What kind of workload you dealing with?

 

[FiberGeek]

That's a much better source lol.

Our workload is just consisting of a VMs that are pretty database heavy, windows domain controllers and employee VMs for remote work.

 

[BlueFox]

Those drives can do 2150MB/s, however that requires using both ports at 12gbit. Single port at 12gbit (or dual 6gbit) would get you 1200MB/s at best and a single 6gbit port means you top out at 600MB/s.

 

[Spartacus]

What pcie is the hba? you might be bottlenecked there too

 

[FiberGeek]

What pcie is the hba? you might be bottlenecked there too

We are going to use an LSI 9300-8i because we're going to getting new servers next summer, and want something that will be future proof...ish

 

[Spartacus]

We are going to use an LSI 9300-8i because we're going to getting new servers next summer, and want something that will be future proof...ish

Should be fine then you should be able to run those drives in raid 1 full/near full speed as long as you use dual link ~ additional details below.

SAS3 is 12Gb/s, or 1500MB/s not accounting for overhead. So you'd never see the full potential of those drives anyway, and even less in a SAS2 configuration with regards to max throughput.

Why would they sell drives that cant reach their rated speed on their connection?

Those drives can do 2150MB/s, however that requires using both ports at 12gbit. Single port at 12gbit (or dual 6gbit) would get you 1200MB/s at best and a single 6gbit port means you top out at 600MB/s.

So first I think your calculations are a bit off for SAS2 6gb dual link is half duplex calculations.
Each link is a full duplex component so it has 6gbit/s x 4 lanes overall = total max of 2400MB/s (with a realistic speed of 2200MB/s).
So SAS2 is just enough throughput for the maximum of those drives assuming raid 1.
Here's LSI's document on SAS and pci-e throughputs: Broadcom Inc. | Connecting Everything

And a couple of table snippets from it:

 

[BlueFox]

So first I think your calculations are a bit off for SAS2 6gb dual link is half duplex calculations.
Each link is a full duplex component so it has 6gbit/s x 4 lanes overall = total max of 2400MB/s (with a realistic speed of 2200MB/s).
So SAS2 is just enough throughput for the maximum of those drives assuming raid 1.
Here's LSI's document on SAS and pci-e throughputs: Broadcom Inc. | Connecting Everything

It's not 4 lanes though. You're thinking multilane connectors like SFF-8087. SAS drives have 2 ports, so, 2 lanes. My calculations are still correct and represent the maximum read or write speeds that would be possible.

 

[Spartacus]

Doesn't that only apply to individual lanes on something like a breakout cable, it should aggregate those lanes via the SAS cables from the HBA/Raid controller to the backplane.

 

[ske4za]

Why would they sell drives that cant reach their rated speed on their connection?

A lot of companies sell things that can't reach their rated speed

But no, they can reach these speeds if your controller supports "wide port" which means combining multiple SAS connections.



And BlueFox is right on both counts as SS530 drives are dual-port, so your speeds are limited to SAS x2.

 

[FiberGeek]

So it sounds like I need to make sure my equipment supports wide port SAS? Is that pretty standard? My google-fu efforts didn't turn up anything useful about that explicitly being supported on most equipment.

 

[Spartacus]

Frankly you have all the equipment anyway right?
Try it out how it is and see if its sufficient performance for your needs.

 

[Evan]

My experience is you won’t generally notice any difference unless it’s a rather heavy workload.
since you have everything just use it
besides I assume the rest of the VM host is already using SAS2 right ?