Our shop took a sip of the the HP C-class blade Kool-Aid in early 2007 and liked the taste of it. I’ve got SQL Server running on a few pieces of HP hardware, and over the next few blog posts, I’ll look at the strengths and weaknesses of the BL460c and BL680c from a DBA’s point of view.
Small, Medium and Large
When I quote hardware price options for a project, I like to give them options for Small, Medium and Large. The proposal will include hardware specs, prices and a ballpark range of the type of load the server can support. In the general case of database platforms, those options would be:
- Small – BL460c – small blade with up to 2 CPU’s, 8 memory slots, 2 HBA’s and 4 NIC’s
- Medium – BL680c – double-height blade with up to 4 CPU’s, 16 memory slots, 2 HBA’s and 4 NIC’s
- Large – DL580 – standalone 4U server with up to 4 CPU’s, 32 memory slots, lots of HBA’s and NIC’s
(I would never quote a project the option of these three different hardware platforms, of course – I would already have an idea of which one they needed, and give them S/M/L quotes for that particular server.)
The C3000 and C7000 Chassis Compared
The core of a blade system is the chassis, and HP’s enterprise chassis for the C-Class is the C7000. The C7000 holds up to 16 half-height blades (or 8 full-height, or some combinations) in a 10u rack space.
In that chassis, we’ve got almost all half-height blades with the exception of one full-height blade. Blades can be mixed in a single chassis, but there’s a dangerous exception that we’ll talk about later.
HP does make a smaller enclosure, the C3000, but I would highly recommend against it except for the most space-challenged shops. The C3000 is 6u tall instead of 10u, but it only supports 8 half-height servers and 3 interconnect bays. Check out the cost comparison using retail prices from Insight:
- C3000 with 2 power supplies and 4 fans – $4,300
- C7000 with 2 power supplies and 4 fans – $6,000
The C3000 might look cheaper, but watch how the cost looks when 2 network switches are added in – after all, the blades need network connectivity:
- C3000 with power, fans, and 2 Cisco 3020’s – $13,900
- C7000 with power, fans and 2 Cisco 3020’s -$15,600
Suddenly, spending the extra $1,700 to get the capacity for 8 more blades seems like a great deal. The real cost on a blade chassis isn’t the chassis itself, but rather the network switches and SAN switches that get plugged into the back side of the cabinet. Speaking of which, let’s take a look at the back of a C7000.
Everything you see in this chassis is the back of a C7000. There’s two rows of fans (at the top and bottom of the chassis), a row of power supplies at the very bottom, and in the middle, the interconnects. This particular chassis has four network switches and two SAN switches. This will seem like a lot of interconnect equipment for just 16 servers, but we tend to only use blades for equipment that needs a lot of connectivity. Examples would be:
- VMware server – each blade needs 4 network ports and 2 SAN ports
- Standalone SQL Server – each blade needs 2 network ports and either 2 SAN ports or 2 iSCSI network ports depending on storage
- Clustered SQL Server – each blade needs 3-4 network ports and 2 SAN ports
Choose Switch Setups Wisely, and In Advance
Here’s where things start to get a little tricky. The half-height blades like the BL460c’s have two onboard expansion slots that take things like network cards or HBA’s. A common configuration might be one dual-port network card and one dual-port HBA. With blades, though, there’s no simple cable to plug into the network card or the HBA. Instead, the connection is handled by the blade chassis, and that connection is hard-coded to specific switches.
If a blade is put in with the cards reversed – like with a network card in a slot that’s connected to a SAN switch – the entire chassis goes into degraded mode. There’s no way to simply reroute the traffic between slots.
This means that if the shop has more than one C7000 chassis, and they want to move a blade from one chassis to another, both chassis must have the exact same switches plugged into the exact same interconnect slots in the back of the chassis. If the shop puts Brocade SAN switches in interconnect slots 3 & 4, then every blade chassis needs to have that same setup. Otherwise, when a blade is taken from one chassis to another, the blade won’t power on, and the chassis will go into degraded mode.
Switch Ports Stay With The Blade Chassis
Blades are so easy to pull out and move around that it’s tempting to whip them all over the place. Because I’m paranoid about uptime, we’ve embarked on a project to balance mission-critical clusters between multiple C7000’s just to make sure they stay up. (We had one instance where we had to take down a C7000 to replace a backplane.)
Unfortunately, however, when a blade server is removed from one slot and popped into another slot, its network port configurations do not travel along with it. SAN ports aren’t an issue since zoning is generally done by the HBA WWN, but network ports don’t have the luxury of zoning by MAC address. C7000 users with the Cisco 3020 switches just have to involve their network staff whenever a blade with special network configurations (like VMware or a cluster heartbeat nic) is moved from one slot to another.
That’s not a defect of the chassis by any means, just something to be aware of.
Choose Blade Arrangements Wisely Too
Earlier I mentioned that a chassis can have a mix of half-height and full-height blades. The chassis uses a series of interlocking shelves to hold each blade up. These shelves must be removed in order to use full-height blades. The problem is that they’re designed in a way that if a full-height blade goes in, more than one shelf must be removed.
In our example photo above, look at the far right slots. We have four half-height blades in a grid. Those four constitute one unit of shelving. To the left of that, we have a full-height blade, neighbored by one half-height blade at the bottom and an empty unit right above that. That group also constitutes one unit of shelving: the full-height slot and its neighbor all have the same shelving setup. That means I could put two full-height blades, or one full-height and two half-heights.
However, if I use two half-heights, and if I remove the half-height blade on the bottom, the half-height blade above it will fall down!
When deciding to use full-height blades, be aware that if only one is used, then one half-height slot next to it at the top will be wasted. Either that, or just decide to never remove the blade on the bottom without removing the one on the top first.
But The Rest Is Positive
Apart from those issues, the HP c-Class blade chassis system has been reliable, easy to manage, easy to service, and a joy to interact with.
I don’t know that the blade setup is necessarily cheaper, especially when given the amount of switching infrastructure, but it’s much easier to grow and manage than a similar number of physical servers.
In my next few posts, I’ll talk about the differences between the BL460c (half-height), BL680c (full-height), and a DL580 standalone server for comparison, focusing on how they impact database administrators.