You simply shift the digits left or right without having to actually perform a calculation.Įvery programming language has an operators for these simple operations, in C-like languages, they are n > m aka shift n right m bits aka divide n by 2^m, and n << m aka shift left aka multiply n by 2^m. That is to say that a computer can perform integer multiplication or division by a factor of two as easily as you can multiply or divide a number by 10. Growing by powers of two is a strictly mathematical concept that makes it easy to take math shortcuts in a computer that's based on two states. You might notice a slight difference in performance benchmarks, but you're very unlikely to notice it unless your computer is doing some heavy lifting and you know what to look for. No version of the drive coding is so much better than the others that you'd notice it in most cases. They all have 128 GB of "raw" space, but they handle the required background stuff differently. Sidenote - there are actually a few ways of writing the required system code, which is why you'll see 120, 124, and 128 GB drives from different manufacturers. The companies making drives have realized that people care more about how big their drive is than whether its usable space is actually a multiple of 2. Now, technically they could slap another chip on the piece to give you 128 GB of "usable" space, but that costs more money. 8 GB is reserved for some specific background management tasks, as stated before. For most drives with 120 "usable" GB of storage, the base drive actually is a 128 GB drive. What they advertise is the "usable" disk space. Simply put, all SSDs are, at base, not what they advertise. This leads to 256GB of raw capacity providing only 240GB of usable capacity, for instance. 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 remapping. Since that part of the silicon doesn't change in size, then the most efficient use of the silicon realestate is to use powers of two in the actual flash capacity. 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. However, SSDs use flash memory, which is always manufactured in powers of two. 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 remapping. Whenever that sector is requested, the drive goes to the new sector, rather than the original sector. As the drive is used, it monitors the sectors (using error correction routines to detect bit level errors) and when a sector starts going bad, it copies the sector to a spare, then remaps it. During initial testing at the factory, any bad sectors are mapped to the spare sectors. The "extra" capacity is held aside to replace bad sectors, so the drives don't have to be perfect off the assembly line, and so that bad sectors can be re-mapped later during use to the spare sectors. The space that’s left is advertised as Gigabytes, netting you 111 Gibibyte in this example.īoth mechanical and solid state hard drives have raw capacity greater than their rated capacity. 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. While I certainly can’t state which exact algorithms need most of that extra space, the calculation stays the same. 128x 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.Īs for as I know, this is something that comes on top of the things already explained above. It has been common practice to leave a bit of space unpartioned-on top of the space that has already been made invisible by the controller-when SSDs first hit the market, but the algorithms have gotten significantly better, so you shouldn’t need to do that anymore.ĮDIT: There have been some comments regarding the fact that this phenomenon has to be explained with the discrepancy between advertised space, stated in GigaBytes (e.g. The preserved space simply gives the controller/firmware room for stuff like TRIM, Garbage Collection and Wear Leveling. If you-for example-bought a 120GB drive, you can be pretty sure that it’s really 128GB internally. While a lot of modern SSDs like the 840 EVO series do provide the sizes you’re used to like, the mentioned 256GB, manufacturers used to preserve a bit of storage for mechanisms fighting performance drops and defects.
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