What is up with the SSD size in the new MBA 128 GB (not the usual idiotic complaint)

[avalys]

Okay, let's review.

Hard drive manufacturers have always used decimal gigabytes (GB) to measure sizes.

Windows and Mac OS prior to 10.5 used binary gigabytes (GiB), resulting in a size discrepancy.

10.6 now uses decimal gigabytes, and is consistent with hard drive manufacturer units. So, my 250 GB hard drive is reported as having 250 GB of total capacity, and 250,000,000,000 bytes (plus some change).

Now - the new MBA 11" 128 GB is advertised as having an 128 GB SSD. However, Mac OS X 10.6 reports it as having a total capacity of 121.3 GB (121,300,000,000 bytes) in "About This Mac". What's up with that?

If 128 GB actually means GB, then that translates to 119.2 GiB. Clearly not the problem (and we know that 10.6 does not use GiB anyway).
If 128 GB actually means GiB, then that translates to 137.4 GB. Clearly not the problem either.

Clearly it is not formatting overhead either, because my 250 GB conventional hard drive is reported as a total capacity of 250 GB, meaning "About This Mac" does not perform any correction for formatting overhead when reporting the total capacity (not that I would expect it to).

What's up here?

 

[neteng101]

Probably is due to "spare" space reserved on SSDs as scratch space for the drive controller to work with. Different manufacturers have different ways of representing SSD space - 128GB is total, 120GB is usable. Eg. Some SSDs are sold as 60GB and some as 64GB and they are typically the same size.

 

[dal20402]

SSDs set aside part of their space to compensate for bad blocks and to make write operations stay smoother for longer. Unfortunately, the manufacturer's specified size is not the size available after formatting, but the total space including set-asides.

 

[avalys]

Ah, good to know, thank you.

 

[jmazzamj]

It's called drive provisioning.
However Apple should not advertise the MBA as having 128 GB drive in it when it only has 120GB drive.