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I bought a MBA with an SSD advertised to be 256GB.

The actual unformatted capacity, according to the system info tool, is only 251000193024 bytes.

enter image description here

This is only 233 Gibi (2^30) bytes, or 251 Giga (10^9) bytes. Regardless of whether Apple means Giga or Gibi in their spec sheets, this capacity is still outrageously lower than advertised.

Is a part of the SSD reserved for some hidden function and is subtracted from the size that appears in the system info?

Or is Apple consciously cheating its customers?

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This has been going on since magnetic and SSD storage was born. Apple isn't any different from other computer sellers and OEM suppliers, they all round the number because different file systems format disks/SSDs differently. Stu has it exactly right below. –  Richard Jun 25 '12 at 21:09
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@Richard I think you missed the point. The quoted capacity is of the unformatted size. It would be interested to hear the justification for calling it 256 when is seems to be only 233, but calling it "rounding the number" is ridiculous.. There are 250GB SSDs out there, so why not round it to 250? ... –  GJ. Jun 26 '12 at 0:59

4 Answers 4

It is interesting to note that Hard Drive Manufacturers use the correct meaning of "Gigabytes" when referring to hard drives, and not what most people think of as a GB which is actually a Gibibyte:

Gigabyte = 10 to the 9 bytes = 1000000000bytes = 1000 megabytes

Gibibyte = 2 to the 30 bytes = 1073741824bytes = 1024 mebibytes

Since Mac OS X also switched to this "proper" definition for storage units such as Gigabytes as of Snow Leopard, then the size of the drive should be understood in these terms, noting that on a Mac the OS and the Hard Drive will be talking the same language, there is no need to do the conversion from Gibibytes to Gigabytes.

The given size should be correct and accurate as reported on a Mac as they have a common understanding of what a Gigabyte is, and incorrect on Windows which uses the commonly misused Gibibyte structure on it's filesystems whilst users incorrectly assume it is measured in Gigabytes.

Technically manufacturers should use GiB not GB when marketting and packaging their drives, but they don't/won't, and use Gigabyte which they know practically everyone uses incorrectly.

In your case the available unformatted capacity is indeed low even for using the correct maths, but as referenced in another answer the size of 256Gb refers to the total storage capacity of the drive, and you have to not only accept that formatted capacity is lower, but also that the capacity that is available to format can also be lower, if portions are reserved for the purposes of replacing bad blocks through the lifecycle of the drive.


Lengthy comment follows after the first 2 comments below - too long for a comment tho, but not really part of the answer other than background info perhaps


The space reservation thing is not something I feel I can be canonical about, I know the theory, I can't really backup it up once and for all, especially because this is an Apple non-consumer drive. However, almost all drives over provision, providing more space than they state in order to have capacity for replacing worn areas.

It's not 100% clear, but it certainly appears to be the case, that in the case of these Apple supplied drives, the provision of "backup" blocks is taken from the stated capacity from the off. I think this is how most manufacturers do it, hence the fact that you see way more 60Gb drives than 64Gb etc. The implication being that there is 64Gb of storage capacity, and 60 of it us user addressable. Whether you call that a 60 or a 64Gb drive could potentially be taken either way. I would call it a 60, but from a legal perspective I would imagine both cases are arguable accurate. As for the instances you see where the available capacity is equal to or greater than the nominal capacity (in the case of the 128Gb drives you see) then it could simply be the case that some manufacturers are either more generous, or more worried about failure, than others and genuinely put in more chips worth, say 132Gb for example.

So in you case, you likley have a 256Gb drive in terms of chips, the fact that the user addressable space is lower, even before formatting is likely just how it is. I have an Apple supplied 128Gb Air SSD which shows much the same behaviour:

APPLE SSD TS128C:

Capacity: 121.33 GB (121,332,826,112 bytes) Model: APPLE SSD TS128C

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+1 if the reservation for replacing bad blocks explains this. However, as @gentmatt above noted, other SSDs don't seem to have any such discrepancy and I'm sure they still manage to replace blocks in a reasonable way over time. So it still doesn't make sense... –  GJ. Jun 26 '12 at 11:08
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This is a really good answer. Just some reference to strengthen the last sentence would be nice. –  gentmatt Jun 26 '12 at 11:17
    
The extra capacity to account for bad blocks is called "Overprovisioning", and is typically ~5% to ~%15 percent of total drive capacity. For what it's worth, NO other manufacturer on the market counts their Overprovisioning towards their capacity. Whatever is going on here, it's not related to the extra space reserved for wear-leveling and bad-block management. –  Fake Name Apr 21 '13 at 0:19

No its a thing that has been going on since are drives were first introduced (when I was young and 20Mb Hard Drives were the whizz bang and DOS 4 was hot, it wasn't such a big deal)

Hard drive manufactures quote their hard drive sizes in thousands, millions, billions, just like cash, in base 10 (GB)

People who use computers calculate size in multiples of 2^10 bytes (1024 bytes)

The difference mostly is the conversion, plus some that is left over from partitioning that cannot be partitioned due to the way that partitioning works, and quite probably the Lion Recovery Partition.

Finally, in answer to your question, does Apple engage in false advertising. No, it does specifically state on all its product pages where appropriate to the device, that the HD capacity is measured in GB.

1GB = 1 billion bytes; actual formatted capacity less.

You should also be aware that some SSD's come with a default space used for formatting, and redundancy for wear levelling and can be up to 5% of the nominal capacity of the SSD drive. This area is managed by the SSD firmware and is never seen by the OS and the OS reports a lower size than the actual SSD size.

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I don't think partitioning, or Lion Recovery, has anything to do with this. GJ's screenshot is from the system report, which shows the device size, not the volume size. My SSD (not from Apple) shows as "128.04 GB (128,035,676,160 bytes)" here, and below that the primary volume is "127.17 GB (127.174,889,472 bytes)", with another "650 MB (650,002,432 bytes)" taken by the "Recovery HD" volume. –  Ken Jun 25 '12 at 20:34
    
A screenshot of the partition tab in Disk Utility might be a better source of data. –  Stu Wilson Jun 25 '12 at 20:48
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@StuWilson so what could the "256" in "256GB" possibly refer to? I certainly don't see 256 billion bytes anywhere. Needless to say that the formated capacity decreases the available size below the 251 billion bytes... also, the partition's size is 249,821,663,232 according to Disk Utility. –  GJ. Jun 25 '12 at 21:03
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Mark: My Samsung SSD doesn't work that way. gentmatt's Crucial SSD doesn't work that way. Wheat's OWC SSD doesn't work this way. I think there's still a legitimate question as to why only Apple seems to claim a higher capacity than they sell. –  Ken Jun 26 '12 at 22:07
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No, it's different than "the old 'formatted capacity will be lower'" -- because that statement applies here, too! (I'm missing about 0.87 GB from formatting and partitioning, though I can see there is 128 GB addressable.) The issue is not that it's not usable, but that it's not even addressable. If no piece of info anywhere (except the marketing materials) say "256", how can you know they're "not inaccurate"? If they lied and put only 250 GB in, what exactly would be different in what we've seen? –  Ken Jun 29 '12 at 14:18

As has you have guessed, this does not have to do with how decimal and binary numeral systems measure a GB.

Apple has released a knowledge base article (see last paragraph) where they mention briefly why the capacity in Solid State Drives is smaller:

Understanding storage capacity in Solid State Drives

Storage capacity displayed in Disk Utility by for Solid State Drives will show a slightly smaller size. For Example, the 256 GB Solid State Drive (SSD) should have a total of approximately 250 GB.

These items may account for the additional space used in your Solid State drive:

  • EFI Partition
  • Restore Partition
  • Wear-leveling blocks
  • Write-buffer area
  • Metadata
  • Spare blocks
  • Grown bad blocks
  • Factory bad blocks

What's surprising is that Apple talks about Solid State Drives and not Flash Memory even though they are basically the same.

I had thought that this is because this article is about SSDs in general. But this can't be true because not all third-party SSDs which you can use in your Macbook Pro use that much space for wear-leveling for example. I have used a 128GB SSD Crucial m4 SSD in my Macbook Pro and the capacity reported by Disk Utility is 128GB.

Technical details about the management of the flash memory in the SSD differ depending on the manufacturer. Therefore I'll think this article refers to Apple SSDs only (which does not mean that some of the points raised here aren't true for other SSDs as well).

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I don't think the typical discrepancy between 1000MB or 1024MB being 1GB is the issue here.

Apple's OS uses the same metric as storage manufacturers (1000MB = 1GB), so a 256GB drive should be reported as 256GB. My 128GB SSD is reported as 128.04GB in OS X's 'System Report'

My guess is that some manufacturers make 250GB drives and others make 256GB drives, and Apple is sourcing from both and just listing both as 256GB.

edit: Space reservations for bad memory cells could also be the possible cause in this case. I know some manufacturers do that.

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