replace controller, add second controller, or use expander?

Question

I have a Proliant DL380 G6 that I am re-purposing as a Hyper-V host for a new, off-site data center that will host our DR services.

The server currently has a P410i controller with the 512MB BBWC module. The drives installed are SFF 6G 10K drives. I plan to add the HP 516914-B21 drive cage, which gives me 8 more SFF drives, bringing the total to 16. To get the additional 8 drives connected, I have one of three choices:

1. Install a new controller that can support 16 drives.
2. Install a second controller.
3. Install a SAS expander, such as the HP 468406-B21 recommended by HP's spec sheet for my server.

My question is: how do I know if I'm going hit a performance ceiling by putting 16 drives on the P410i or using the expander? And if I am, how do I select an appropriate controller? I'm not sure what specifications I should be looking at.

Solution

A few items... You obviously will not be able to address all of them with your existing P410 controller, but:

*Do you need all 16 drives in one array?*

I ask, because there isn't much utility in having that many disks in a single array unless you know your read/write patterns and are designing around it.

If you don't need all 16 disks in a single array, you're free to obtain a second Smart Array P410 RAID controller to address the second drive cage. That is the simplest solution.

There aren't any HP controllers that support the setup you're looking for without an expander. The Smart Array P812 controller is meant to accommodate internal drives, but is really intended to fan-out to external enclosures (D2600 and D2700, for instance).

All other options internally would require the use of the HP SAS expander. I use the expander in a few installations and have had good luck with it (once the firmware is current). If you have an old version or an old firmware, the expander only works at 3G. However, it can be a bit of a bottlneck, depending upon what you're doing (e.g. an array of SSD's).

The expander is a 6G SAS unit, so you're looking at a single 4-lane SAS SFF-8087 connection from your onboard Smart Array to the expander. There's an option to use a second connection... So that's either 24 or 48Gb/s... From the expander, you'll have 2 SFF-8087 cables going to each drive cage.

16 x 6G disks == 96Gb/s theoretical max. You'll be oversubscribed on your link to the controller by either 2x or 4x, depending on how you cable the chassis.

Let's assume your single disk is capable of real-life ~200 Megabytes/second transfers. Maybe slightly-lower: 1.5Gbps...

1.5Gbps x 16 = 24Gbps

That fits well with a single 4-lane 6 SAS connection. Even better if you use dual connections between the controller and the SAS expander.

So yeah, it's possible to stuff a DL380 G6/G7 with disks and get good performance with the expander. I think the separate P410 controller is a smoother solution, though.

Hardforum.com has an extensive resource for the HP SAS expander.

Comments

[ewwhite]

Nice question... will answer in detail in a few...

[the-wabbit]

Mmmh, why does this look so very much like my office space?

[longneck]

Thank you for the thorough analysis. I think I will go with the P410 since I intended to have 2 RAID5 or RAID6 arrays anyway. And compared to the cost of the drives, buying the controller instead of the expander is nothing.

[ewwhite]

Since this is a DR site, performance isn't too big of an issue... But in general, I usually recommend against RAID 5/6 for virtualization hosts.