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Discussion in 'MacBook Pro' started by 7DSniper, May 15, 2012.
Why do SSD have odd sizes? e.g. 64, 128, 256, 512, why not 100, 300, 500, etc;?
As you can see, they are multiples of 2, like the RAM in your computer (1 GB, 2 GB, 4 GB, 8 GB, 16 GB, ...), and since they share the same technology roots, it probably comes from there.
But then again, the above is only a guess, as there are also 180 GB, 240 GB and 480 GB SSDs out there.
And HDDs also had some odd sizes, 30 GB, 60 GB, 120 GB, 160 GB, 250 GB, 320 GB, 750 GB and so on.
Sim is on the right track. Generally we work in a base 10 world (everything is a power of 10). We are used to hearing things that way A MB is 1000 bytes (when in actuality it is 1024). The computer works on base 2 (powers of 2) as bytes can only be 2 states on or off. Recently in the last few years due to changes in advertising and complaints from consumers about not getting the storage promised (as when a disk is formatted, the sector and block size changes the available sizing to the system). Lots of SSDs are following the Base 2 sizing, and the strange sizing also has to do with the over provisioning that the controller uses to create space allocated for controller operations, such as caching, and processes involved with the Garbage collection and wear handling operations. In reality the drives are actually larger drives than they report, but a portion is reserved for controller usage.
I know this description makes things clear as mud right??
HDDs have been reported simply in base 10.
240GB SSD actually has 256GB. A 480 has 512GB of actual NAND. 100 is 128 GB flash and so on. Yet only the smaller number is the actual user addressable space.
And those are still usually base 10 reports. As 180GB usually means 170GB in base 2. All the rest is just spare area for wear leveling.
Some actually now report in real base 2 like all those that come in 128, 256, 512 GB sizes.
So how is over-provisioning figured on the SSDs that use base 2 (like Crucial)? I have a 256GB drive and iStat pro reports 159.2GB used and 96GB free. That can't include the blocks reserved for over-provisioning.
iStat Pro uses base 10 numbers. You have 249.2 GB capacity on your SSD using base 2.
Modern SSDs are using the 25nm 8GB die described here. So the drive capacity will be however many of these 8GB die are soldered to the PCB inside the SSD.
For example, here is a recent review of a new Corsair drive. Look at the chart below from the review.
Note for the 128GB drive we have 16 8GB die for a total of 128GB raw capacity. Ditto for the 256GB... 32 X 8GB = 256 total.
The short answer to the OP's question is it is driven by multiples of an 8GB NAND die size.
Most SSDs are "over provisioned" like the Corsair in this example. So the Corsair has 119.2GB usable space (rounded to 120GB) out of 128GB. Some vendors are advertising these drives as 128GB drives when they may really only have ~120GB usable space. Other vendors are more accurately advertising the usable space number... 120GB in this case.