Windows® Internals, Sixth Edition, Part 2

The most important difference between flash memory and RAM is that RAM can be read and written an almost infinite number of times, while flash memory can be overwritten something less than 100,000 times. (Depending on the type of flash memory, it may be as few as 1,000 times). In effect, flash memory wears out, so flash memory should be treated more like media with a limited lifetime (such as a floppy disk) than RAM or a magnetic disk. Another major difference between flash memory and RAM is that flash memory cannot be updated in place; a block must be erased before it can be written (even for NOR-type flash memory). Flash memory is significantly faster than magnetic disks (usually by a factor of 100,000, or so; access time: 50 nanoseconds versus 5 milliseconds), but it is slower than RAM (usually by a factor of 50). From a practical perspective, memory access time is not the whole story because flash memory is not on the system memory bus. Instead, it sits behind a disk-type controller interface on an I/O bus, so in reality the difference between flash and magnetic disks may be on the order of only 1,000 times faster, and in some workloads a rotating magnetic disk can outperform a low-end SSD.

NAND-Type Flash Memory

NAND-type flash memory is most commonly used in SSDs, so that is what we will examine in detail. NAND-type flash comes in two types:

Single-level cell (SLC) stores 1 bit per internal cell, has a higher number of program/erase cycles (on the order of 100,000), and is significantly faster than multilevel cell (MLC), but it is much more expensive than MLC.

Multilevel cell (MLC) stores multiple bits per internal cell and is significantly cheaper than SLC. MLC needs more ECC bits than SLC, has fewer erase cycles (~5,000), and consumes more power than SLC.

NAND-type flash is typically organized into 4,096-byte pages (which may be exposed as eight 512-byte sectors or a single 4,096-byte sector), which are the smallest readable or writable units, and the pages are grouped into blocks of 64 to 1,024 pages, with thousands of blocks per chip. As with a magnetic disk, there is overhead on each page, with ECC, page health, and spare bits. The block is the smallest erasable unit, so to change a single sector within a page requires that the entire block be erased and then rewritten. (Flash cells can be written only after they have been erased.) This means that writing a sector to an empty block is very fast, but if there is not an available empty block, the controller has to perform the following actions:

Read the entire block into the controller’s internal RAM.

Erase the block in the flash memory.

Update the block in RAM with the contents of the new sector.

Write the entire block to the flash memory.

Notice that what started as a write to a sector (512 bytes) became a write of an entire block. For this example, if we assume 128 pages in a block and a completely full block, then the write would take 1,023 times longer (the block contains 1,024 sectors) than the write of a single sector to an empty block. This example is a worst case and is decidedly not the norm, but it illustrates an important aspect of SSDs: as more and more of the SSD’s memory is consumed, it will have to rewrite substantially more data than a single sector. In effect, SSDs slow down as they fill up. This has important implications that are addressed in the next section, File Deletion and the Trim Command.

As a block wears out, eventually it will fail to erase. Also, the more a block is erased and rewritten, the slower it becomes (a result of the physics behind how flash memory is implemented). This means that an SSD will only get slower as you use it—even on an empty block. For example, on a 1-GB USB MLC flash disk with 128 pages per block (giving us 2,048 blocks), erasing and writing one block per second would wear out all the blocks in 23.7 days (assuming a maximum of 1,000 erase cycles per block, which is typical for the cheaper flash disks). Erasing and writing the same block once per second will wear out that block in only 16.6 minutes! SSDs typically have spare blocks held in reserve (often 20 percent of the SSD’s capacity) so that if a block wears out, the data is moved to a spare block. Clearly, flash memory cannot be used the same way as RAM or a magnetic disk.