SSDs seems to come in quite a variety of ‘new’ sizes these days, but why is that? Today’s SuperUser Q&A post has the answers to one curious reader’s question.
Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.
Photo courtesy of Jung-nam Nam (Flickr).
The Question
SuperUser reader Dudemanword wants to know why SSDs seem to come in weird GB sizes:
Why do companies manufacture SSDs in what seem to be “non-standard” sizes?
The Answer
SuperUser contributors Patrick R. and Adam Davis have the answer for us. First up, Patrick R.:
Followed by the answer from Adam Davis:
If you, for example, bought a 120 GB drive, you can be pretty sure that it is really 128 GB internally. The preserved space simply gives the controller/firmware room for stuff like TRIM, Garbage Collection, and Wear Leveling. It was common practice to leave a bit of space unpartitioned – on top of the space that had already been made invisible by the controller – when SSDs first hit the market, but the algorithms have gotten significantly better, so you should not need to do that anymore.
EDIT: There have been some comments regarding the fact that this phenomenon has to be explained with the discrepancy between advertised space, stated in Gigabytes (i.e. 128 x 10^9 Bytes) versus the Gibibyte value the operating system shows, which is – most of the time – a power of two, calculating to 119.2 Gibibyte in this example.
As for as I know, this is something that comes on top of the things already explained above. While I certainly can not state which exact algorithms need most of that extra space, the calculation stays the same. The manufacturer assembles an SSD that indeed uses a power of two number of flash cells (or a combination of such), though the controller does not make all that space visible to the operating system. The space that is left is advertised as Gigabytes, netting you 111 Gibibyte in this example.
Have something to add to the explanation? Sound off in the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.
On mechanical drives, they can add arbitrary amounts of spare storage since they control the servo, head, and platter encoding, so they can have a rated storage of 1 terabyte with an additional 1 gigabyte of spare space for sector re-mapping.
However, SSDs use flash memory, which is always manufactured in powers of two. The silicon required to decode an address is the same for an 8 bit address accessing 200 bytes as an 8 bit address accessing 256 bytes. Since that part of the silicon does not change in size, then the most efficient use of the silicon real estate is to use powers of two in the actual flash capacity.
So the drive manufacturers are stuck with a total raw capacity in powers of 2, but they still need to set aside a portion of the raw capacity for sector re-mapping. This leads to 256GB of raw capacity providing only 240GB of usable capacity, for instance.
