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NAS - hard drive and transfer speeds

Okay, so I've got the general idea of what I want, now it is time to hash out some of the specs. Where to start?

The working parts of the device should be modern or at least based on modern standards. Since I will be going to all the trouble of building this device from scratch, I don't want to be put into a position where a significant redesign is needed in the event of a hardware failure of some kind.

• The primary hard drive interface should be SATA. The old PATA (a.k.a. IDE) hard drive standard isn't gone yet, but it is certainly fading away into the sunset, not to mention its technical inferiority. I won't rule out PATA completely for one or maybe two drives if the motherboard provides it. I'll need to be more careful when integrating PATA drives into the RAID configuration to prevent their technical limitations from killing the performance of the entire device.
• The motherboard would ideally use the ATX power connector standard. This dramatically increases the number of power supply options, as ATX is the most common standard available.
• The motherboard needs to support multiple SATA hard drives. The preferred number is 6 on-board connections, but if this is not possible in a small, power efficient machine, it should at least be upgradeable via an expansion card or two.

Generally speaking, a NAS device doesn't require much processing power. The largest bottleneck in performance will most likely be the network itself. Most home networks will use wired fast Ethernet (100 Megabits/sec), wired gigabit Ethernet (1000 Megabits/sec), wireless g (54 Megabits/sec), or wireless n (248 Megabits/second). Note, however, that these measurements use the bit as the base unit of measure, not the byte (8 bits) that most of us are comfortable with.

Fortunately, Wikipedia has an excellent collection of device bandwidths available at http://en.wikipedia.org/wiki/List_of_device_bandwidths that shows data transfer rates in both bits and bytes. It also appears to be a fairly complete list of device interfaces that may be used in this system.

One important point to keep in mind is the perceived performance of a NAS device will be effected by the slowest point in the data transfer chain. This includes the network adapter on the machine using the storage device and everything between the two machines. I've configured my home network to support gigabit transfer rates, so my NAS device should take full advantage of this. This means I should be able to get a theoretical top transfer rate of 1000 Mb (megabits) per second, which is equal to 125 MB (megabytes) per second.

Here is a breakdown of possible hard drive transfer rates for this device. SCSI is a bit out of my price range and overkill for a home solution.

1. Ultra DMA ATA 66 - 528 Mbit/s = 66 MB/s
2. Ultra DMA ATA 100 - 800 Mbit/s = 100 MB/s
3. Ultra DMA ATA 133 - 1064 Mbit/s = 133 MB/s
4. SATA 150 hard drive - 1500 Mbit/s = 187.5 MB/s
5. SATA 300 hard drive - 3000 Mbit/s = 375 MB/s

The last three interfaces are all faster than the maximum transfer rate possible over a gigabit network and should be suitable for such an application. As stated before, other technical merits (hot pluggable, no restrictions on writing to multiple devices at once, newer standard with stronger future) make SATA the ideal option. In the current market, you would be hard pressed to find large SATA hard drives that don't conform to the 300 standard, and the price differences between 150 or 300 aren't significant. You are more likely to find disk controllers on the motherboard or expansion cards (e.g. PCI cards) that use the slower SATA 150 interface. Fortunately, virtually all SATA 300 hard drives are backward compatible with the SATA 150 controllers, so compatibility is virtually a non-issue.

Speaking of expansion cards, it is quite likely one will be needed in my device. This adds a new interface to take into account when assessing bandwidth bottlenecks. According to the Wikipedia page, the 32 bit PCI expansion slot running at 66 MHz (the most common kind of expansion slot today) has a theoretical maximum transfer rate of 2133 Mbit/s, or 266.7 MB/s. This means that one should not expect the best performance from a single SATA 300 or multiple SATA 150 drives connected to such an expansion card. Still, for the needs of a NAS device, a PCI expansion card supporting two SATA 150 drives should work nicely for expanding a RAID configuration. Possibly using a PATA style configuration (pairing a motherboard controller with a PCI controller for RAID1 mirroring arrays) would optimize reads and writes when dealing with large files.

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