One of the functional limitations of SSDs is while they can read and write data very quickly to an empty drive, overwriting data is much slower. Reading the proper value out of the cell requires the memory controller to use a precise voltage to ascertain whether any particular cell is charged.
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With TLC NAND, the cell can have eight values, and QLC has 16. With MLC NAND, the cell may have four values - 00, 01, 10, or 11. With SLC NAND, the controller only needs to know if the bit is a 0 or a 1. The reason TLC NAND is slower than MLC or SLC has to do with how data moves in and out of the NAND cell. “If I had asked people what they wanted, they would have said faster horses.” - Henry Ford The problem is, even the fastest spinning drive with the largest caches and smallest platters are still achingly slow as far as your CPU is concerned. Western Digital’s 10,000 RPM VelociRaptor family is the fastest set of drives ever built for the consumer market, while some enterprise drives spun as quickly as 15,000 RPM. The hard drive industry introduced smaller platters, on-disk memory caches, and faster spindle speeds to counteract this trend, but there’s only so fast drives can spin. One millisecond is 1,000,000 nanoseconds, and it typically takes a hard drive 10-15 milliseconds to find data on the drive and begin reading it. Latency in HDDs is measured in milliseconds, compared with nanoseconds for your typical CPU.
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If a drive is asleep or in a low-power state, it can take several seconds more for the disk to spin up to full power and begin operating.įrom the very beginning, it was clear that hard drives couldn’t possibly match the speeds at which CPUs could operate. The drive may need to read from multiple locations in order to launch a program or load a file, which means it may have to wait for the platters to spin into the proper position multiple times before it can complete the command. Because the drive heads must align over an area of the disk in order to read or write data, and the disk is constantly spinning, there’s a delay before data can be accessed.