In the case of a 512 GB drive, the EFI and recovery partitions are only 850 MB. Now, "only" 2.3% spare area is a bit low for a typical drive (a Samsung 840 Pro has around 7% spare area) so the usual tidbit about not filling the drive more than, say, 80-85% is useful. The extra 12 GB are for over-provisioning (scratch space, etc.) per this article So 512 GB = 512 billion bytes according to Apple, but what you actually get is 500 billion bytes. As everyone wrote, the general trip up is that there is unfortunately still confusion between SI and binary prefixes, in part because there is still a refusal by a lot of the tech industry to use them properly.Ĭapacity: 500.28 GB (500,277,790,720 bytes)
Actual hardware over provisioning for the purpose of controllers handling write balancing, cell death and so on is hidden, the full advertised amount should be available.
That's irrelevant to this discussion though. Anandtech did some good investigation about it and has made it a standard part of their SSD reviews starting around their "Exploring the Relationship Between Spare Area and Performance Consistency in Modern SSDs" article at the end of 2012. It's not something that most users need to worry about, but it's there. For most modern consumer-targeted SSDs, additional manual over provisioning of 25% or so can dramatically improve IOPS and performance consistency. Otherwise you can let the system and disk handle any over provisioning or redundancy. Nowadays the only reason to keep free space is to ensure that applications that need to write to disk can. You're right, but that argument is also completely irrelevant when considering SSDs. That's just wrong, and is especially aggravating since storage is now measured in SI-prefix gigabytes so you're actually only getting 112 imperial GB's worth of storage, minus however much space you're willing to sacrifice for performance. To the OP: I was sure that Apple sold these as having 120GB capacity, but you're right, they claim 128GB (with a bullshit "actual formatted capacity less" disclaimer). On the plus side, this redundancy is shared by the hardware and OS. It's not a hard requirement: everything will work even when filled to capacity, it'll just be slower.
Keeping 10-15% space free on your disk helps the filesystem keep fragmentation down, and the more unused space you have the easier it is for the flash controller to maintain good performance. You'll still want some space, as various temp files and other things need to be created during normal system operation. With the memory compression in Mavericks, much less paging space is needed, so you may be able to cut that a bit closer. A 10% reservation for the system was mainly desirable to accommodate memory paging.
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