Hawaii Linux Institute wrote: ...
Second, the access time of flash memory is measured in nanoseconds, thus, there is no latency compared to hard disc, which typically has an access time measured in miniseconds.
Milliseconds you mean. But I keep forgetting about this BIG one. In situations where sustained throughput is low, but random access are high, CF cards may actually be FASTER. A flash read cycle always takes the same amount of time (generally on the order of 30-120ns for NOR flash, not sure about NAND whcih many of the larger CF cards may use), no matter where you're accessing relative to where you just accessed. Hard drives are great for sequential access, but really suck on highly random small reads (say only a few bytes) because they have a moving head that has to seek. Seek times for consumer IDE hard drives seem to be running around 5-20ms these days.
Some new generations of USB 2.0 compliant compact flash sticks can write faster than read (and can read and write simultaneously). While the write speed is currently maxed out below 10 MB/s, the technology is evolving very rapidly. If a market exists, a single channel USB 2.0 compliant CF disc may reach 60 MB/s. This is similar to the speed of ATA/66. Eventually, who knows, there may be multiple-channel CF discs. And that will be a totally different story.
I'd like to know how this is technologically possible. Assuming NOR flash (I will admit up front that NAND flash may be MUCH faster) with a total cycle time (including address set up and data read strobe) of 30ns (an estimate, but the range is big, I think this is on the low end) with a 32bit wide data bus (mind you, the CF data bus is only 12 bits wide), and assuming that the bus and CF cycles are always in sync (they're not), we have a max theoretical of abotu 128MB/sec. This is assuming a pretty fast chip with no overhead and NO ERASE CYCLES (which take a LONG time). Writes generally involve erase cycles (unless they've gotten smart and erase things during idle time). It is posisble, but VERY unlikely that you would hit 60MB/sec. Are you sure this isn't the theoretical bandwidth of the bus? I'm not denying that it exists, just questioning the feasbility of it given current flash technology). I mostly work with just the ATA ones, not the USB ones as well (because they're cheap, and I'm using them as ATA devices :).
Wayne
--MonMotha
