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Thermal Design

The Pod uses six 120mm (~4.7") fans in a push / pull arrangement to keep the drives cool. Backblaze had a goal of keeping the ambient temp in the Pod to 50°C. The reasoning was that while most drives are rated for operation at 60°C or higher, power supplies are typically rated to only 50°C.

Backblaze says that drive temperature sensors typically report under 40°C, which means ambient is running lower, so the design goal was easily achieved. In fact, Backblaze said they found that they could run with only one or two fans and still achieve their thermal design goals. But the fans draw relatively little power and since they have a relatively high failure rate, using six fans provides a reliability boost through redundancy.

The fans blow from the front of the case to the rear (where the power supplies are), so that power supply heat isn't blown onto the drives.


NAS DIYers frequently spend a lot of time agonizing over the choice of motherboard. But since NASes don't demand a lot from a motherboard in terms of features, it's more cost, form factor and CPU support that tend to be the biggest influencers.

One factor that didn't influence Backblaze's choice of an Intel BOXDG43NB LGA 775 G43 ATX Motherboard, was the onboard SATA ports. The Storage pod actually doesn't use any onboard SATA because, despite Intel's claims of port multiplier support in their ICH10 south bridge, Backblaze noticed "strange results" in their performance tests. Instead they relied on SATA controller cards and SATA expansion backplanes to support the 45 SATA drives (more shortly).

Backblaze didn't have any exotic memory requirements either, so two DIMM slots holding a total of 4 GB of DDR2 800 RAM was fine. For cost reasons, however, they did compromise on the number of PCIe x1 slots. This led them to having to use both PCIe and PCI SATA controllers.


Speaking of SATA, Figure 4 shows the SATA subsystem, which is composed three two-port Syba SD-SA2PEX-2IR PCI Express SATA II controller cards and one Addonics ADSA4R5 4-Port SATA II PCI controller card. Each of nine SATA cables connect to a Chyang Fun Industry (CFI Group) CFI-B53PM 5 Port SATA backplane.

SATA connections (Courtesy Backblaze)

Figure 4: SATA connections
(Courtesy Backblaze)

The card and backplane choices were limited to products using Silicon Image devices. In Backblaze's view, Silicon Image pioneered port multiplier technology, and Backblaze feels that their chips work best together. The port multiplier backplanes use a Silicon Image SiI3726, the SYBA cards use a SiI3132, and the Addonics card has a Silicon Image SiI3124.

I asked whether there was a performance hit from using the PCI-based card and Backblaze confirmed that the PCI-based card did yield lower performance than the PCIe cards. But with an overall throughput of 25 MB/s, the Addionics card was still fast enough for Backblaze's needs. They noted that with 25 MB/s throughput, 2 TB of data can be written in a day and a Pod filled within a month.


As you might suspect, hard drive selection was very important. Backblaze chose Seagate 1.5TB Barracuda 7200.11 (ST31500341AS) drives because they were "more stable" in RAID arrays in their mass testing than WD equivalents and had good pricing for their capacity.

They said they also tested Samsung and Hitachi drives and they also worked well. But the Seagates 1.5 TB drives had the best price/capacity/density combination.

Backblaze also said that all new drives get put through a "pounding" during qualification. And that "every single drive deployed" goes through a burn-in test. They standardize only on a drive type and don't try to match (or mix) lots, firmware revisions or manufacturing dates.

The non-critical (to performance at least) OS drive is a WD Caviar 80GB 7200 RPM IDE (WD800BB).

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