IPhones capabilities

Where did the gigabytes I bought go to?

When choosing a new computer or mobile device, we are faced with a large list of technical product specifications, with model information and clock processor, amount of memory, etc. which may not mean much to many people. But one characteristic that we always verify is the storage capacity, to make sure that it will be enough to store the files we currently have and those we intend to have in the near future.

IPhones capabilities

After starting to use their new device, however, many people eventually realize that the space available for storing personal files such as photos, music, videos and documents – so-called “user files” – is less than the nominal storage space that appears in the specification sheet. But why this happen?

Capacity graph on smartphonesInfographic we originally published in January 2014.

(There’s even troll suing because of that …)

This difference exists due to several factors.

The first is that a storage device – whether magnetic disks (HDDs) or flash memory chips (SSDs) – must first receive a system with one or more partitions, and these must be provided with a structure where files can be stored. They are necessary for the operating system to locate the files and obviously take up a small space, so the space available to a partition is always slightly less than the total capacity of the drive.

OS X Disk Utility

The second factor is that, to work, computers (and mobile devices) come with an operating system pre-installed and it is made up of several files. These system files need to be stored and take up part of the available space on the partition.

OS X System folder

But there’s no use in a computer with only an operating system, right? Computers and mobile devices come with several applications pre-installed, and we usually install many others that, like the OS, are composed of one or more files, which occupy more parts of the space.

OS X Applications folder

And after discounting the space occupied by the storage systems, the operating system and the applications, we finally have some space available for the user’s files. This varies from device to device, depends on the total storage capacity, the operating system – and the space occupied by it can change from one version to another – and the installed applications.

But two other common sources of difference between the nominal capacity of storage devices and the capacity available to the user remain to be considered.

Many computers today use solid state drives (SSD) based on NAND flash memory as storage, as access to them is much faster and is much more reliable than hard disk drives (HDD). This is the case for all notebooks in the current Apple lineup (MacBook, MacBook Air and MacBook Pro) and Mac Pro. IMacs and Macs mini can also be configured to come with SSD or Fusion Drive (which combines both technologies).

The space of SSDs is divided into blocks, each occupying a small area on a chip made of silicon. It happens that, due to imperfections in the silicon, some of these blocks have a service life well below the average, which over time would decrease the capacity of the unit and corrupt the files stored in these blocks. To avoid this, SSDs come with part of the blocks reserved to replace those that are “dying” in the useful area of ​​the unit.

Some manufacturers define the nominal capacity of the unit as the total capacity of the useful blocks, after discounting the reserve blocks. 120GB SSDs, for example, actually have 128GiB = 137.44GB of flash memory with 17.44GB (12.7%) reserved for replacement, while 240GB and 250GB SSDs generally have 256GiB of flash with 12.7% and 9 % reserved, respectively. This is not a problem, on the contrary, it is very good as it guarantees a long service life for SSDs. The problem is that some manufacturers, including Apple, sell SSDs at full capacity, without discounting the capacity of backup blocks – which often confuses users, who will never see all 512GB of their SSD reported in the operating system.

MacBook Pro SSD with Retina displayThe MacBook Pro SSD is sold as a 512GB, but it only has 500.28GB useful.

The 512GB SSD of the MacBook Pro in question is actually 512GiB (continue reading to understand this unit of measure), that is, 549.76GB – 49.48GB of which are reserved blocks, 1.2GB are occupied by the file system , 9.79GB are occupied by the operating system and 58.98GB are occupied by currently installed applications, thus leaving only 430.31GB for user files.

That’s all we need to know when using a Mac. Some operating systems, however, display the computer’s storage capacity incorrectly, which often confuses the user. This problem affected OS X up to version 10.5 Leopard and was corrected with the introduction of 10.6 Snow Leopard, but it is present in all versions of Windows and even iOS.

Take for example an iPhone 6s with 128GB. When checking your ability to Settings »General» About, we will actually see 114GB:

IPhone Capability in Settings

And through iTunes on the Mac:

IPhone capacity through iTunes

Interestingly, even on OS X, iTunes shows the wrong data – probably because it pulls it directly from iOS and to avoid even more confusion regarding what the user sees on their iPhone.

Units of measure

THE byte is a unit of measure of very small amount of information, so it is very common to use prefixes such as kilo, symbol k, which is worth a thousand or 103, and express a value in kilobytes, as we do with units of the International System of Units (SI) such as meter and kilometer, gram and kilogram, etc. Other prefixes multiply units by even greater values, such as mega (M), which is worth a million or 106, jig (G) is worth 109, will have (T) is worth 1012, and so on.

The problem is that some operating systems, such as iOS and Windows, use other prefixes – base 2 instead of base 10. One kibibyte, with KiB symbol, worth 210 = 1,024 bytes. Other prefixes are the mebi (Mi) worth 220, comic (Gi) is worth 230, tebi (Ti) is worth 240, etc. Below we present a conversion table between values ​​expressed in decimal prefixes (base 10) defined by SI, and binary (base 2) defined by IEC (International Electrotechnical Commission).

1 Ki = 1,024 k 1 k ≈ 0.9766 Ki
1 Mi ≈ 1,049 M 1 M ≈ 0.9537 Mi
1 Gi ≈ 1,074 G 1 G ≈ 0.9313 Gi
1 Ti ≈ 1,100 T 1 T ≈ 0.9095 Ti

That alone would be enough to confuse most users, who are used to the decimal system. The biggest problem is that, despite informing storage capacities and file sizes using binary prefixes, these OSs use the symbols of traditional decimal prefixes. The useful capacity of the iPhone in our example can be expressed as 122GB or 114GiB, but what we see on iOS is “114GB”, therefore, false information.

Some operating systems use binary prefixes and appropriate symbols, such as Ubuntu, one of the most widely used Linux distributions on consumer desktop systems.

For those who want to go deeper into the subject (in English), I recommend this Wikipedia article and this Apple support article (the Portuguese version of it is badly translated).