A Primer for Home NAS Storage Speed Units and Abbreviations

January 26, 2019

One of the most common mistakes/confusion I see with regard to storage is how speed is measured.

In tech, there’s some cultural conventions to which units speeds are discussed in.

In the networking world, we measure bits per second

In the storage and server world, we measure speed in bytes per second

Of course they both say the same thing, just in different units. You could measure bytes per second in the networking world and bits per second in the server/storage world, but it’s not the “native” method and could add to confusion.

For NAS, we have a bit of a conundrum in that we’re talking about both worlds. So it’s important to communicate effectively which method you’re using to measure speed: bits of bytes.

Generally speaking, if you want to talk about Bytes, you capitalize the B. If you want to talk about bits, the b is lower case. I.e. 100 MB/s (100 Megabytes per second) and 100 Mbit or Mb (100 Megabit per second).

This is important, because there a 8 bits in a byte, the difference in speed is pretty stark depending on if you’re talking about bits per second or bytes per second. Examples:

200 Mb/s is written to mean 200 Mega bits per second

per second 200 MB/s is written to mean 200 Megabytes per second

Again, the speed difference is pretty stark:

200 Mb/s (Megabits per second, about 1/5th of the total rate available on Gigabit Ethernet) = 25 Megabytes per second

200 MB/s (Megabytes per second, almost double what a Gigabit Ethernet links could send) = 1.6 Gigabits/second

200 Mb/s easily fits in a Gigabit Ethernet link. 200 MB/s is more than a Gigabit Ethernet link could handle.

Abbreviations

It’s generally acceptable to write bits per second as Xb, Xbit, Xbit/s, and Xbps, where X is the multiplier prefix (Mega, Giga, Tera, etc.)

The following are examples of 1.21 Gigabits per second :

1.21 Gbps

1.21 Gb/s

1.21 Gbit/s

It’s generally acceptable to write bytes per second as XB, XByte, XByte/s, and XBps, where X is the multipler (Mega, Giga, Tera, etc.)

The following are examples of 1.21 Gigabytes per second:

1.21 GBps (less common)

1.21 GB/s

1.21 GByte/s

A Gigabit Ethernet interface can theoretically handle 125 MB/s (1,000 mbit / 8 bits per byte = 125). A 10 Gigabit Ethernet interface. Depending on your NIC, horsepower, and systems, you may or not be able to reach that. But that’s the theoretical limit for Gigabit Ethernet.

10 Gigabit Ethernet (10GE) can theoretically handle 1250 MB/s (10,000 mbit / 8 bits per byte).

Binary Multipliers

There’s also KiB (Kibi Byte) and Kib (Kibibit), where kibi is a 1024 multiplier, and not 1,000. GiB (GibiByte) and TiB (TibiByte) are 10242 and 10243, respectively.

The idea is to be native to the binary numbers, rather than multiples of 10 (decimal).

We don’t tend to use those measurements in network or storage transmit/receive rates, but it’s showing up more and more in raw storage measurements.

Overhead

SATA I, II, and III are 1.5, 3, and 6 Gigabits/second respectively. They push 150, 300, and 600 MB/s respectively. You’ll probably note that math doesn’t check out: 6 Gigabits/second divided by 8 bits in a byte is 750 MB/s, not 600 MB/s, so where did the extra 150 MB/s go? I’ll cover that in the next article.