Flash (2)

Flash uses a very different architecture and has different properties than standard DRAM. The most important differences are:

1. Reads to Flash are sequential and read an entire page, which can be 512 bytes, 2 KiB, or 4 KiB. Thus NAND Flash has a long delay to access the first byte from a random address (about 25 μS), but can supply the remainder of a page block at about 40 MiB/s. By comparison, a DDR4 SDRAM takes about 40 ns to the first byte and can transfer the rest of the row at 4.8 GiB/s. Comparing the time to transfer 2 KiB, NAND Flash takes about 75 μS, while DDR SDRAM takes less than 500 ns, making Flash about 150 times slower. Compared to magnetic disk, however, a 2 KiB read from Flash is 300 to 500 times faster. From these numbers, we can see why Flash is not a candidate to replace DRAM for main memory, but is a candidate to replace magnetic disk. 而且似乎cache的block size大一点好。
2. Flash memory must be erased (thus the name flash for the "flash" erase process [电影“黑衣人”。威尔史密斯是条汉子。]) before it is overwritten, and it is erased in blocks rather than individual bytes or words. This requirement means that when data must be written to Flash, an entire block must be assembled, either as new data or by merging the data to be written and the rest of the block's contents. For writing, Flash is about 1500 times slower then SDRAM, and about 8–15 times as fast as magnetic disk.
3. Flash memory is nonvolatile (i.e., it keeps its contents even when power is not applied) and draws significantly less power when not reading or writing (from less than half in standby mode to zero when completely inactive).
4. Flash memory limits the number of times that any given block can be written, typically at least 100,000. By ensuring uniform distribution of written blocks throughout the memory, a system can maximize the lifetime of a Flash memory system. This technique, called write leveling, is handled by Flash memory controllers.
5. High-density NAND Flash is cheaper than SDRAM but more expensive than disks: roughly $2/GiB for Flash, $20 to $40/GiB for SDRAM, and $0.09/GiB
6. for magnetic disks. [这本书比较老了] In the past five years, Flash has decreased in cost at a rate that is almost twice as fast as that of magnetic disks.

Like DRAM, Flash chips include redundant blocks to allow chips with small numbers of defects to be used; the remapping of blocks is handled in the Flash chip. Flash controllers handle page transfers, provide caching of pages, and handle write leveling [使平坦].

六级/考研单词: differentiate, thereby, random, data, assemble, merge, standby, nil, expense, decrease, redundant, defect