Thanks, you're right. It should be milliseconds, not miniseconds. :-)
The technology on flash memory is evolving so rapidly, no one really
knows what we could expect. For example, go to the USPTO web site and
do a quick and dirty search on "flash memory" and "Liauh", and you will
find at least 11 patents. There are always those little bitty things
that are constantly invented to improve the read, and more particularly,
write, speed. Remember 7 or 8 years ago when we thought the EDO DRAM has
reached the possible speed limit of dynamic memory cells? Current flash
memory is almost at the same stage of the EDO DRAM.
BTW, I did wrong calculation in the linear speed of a hard disc. It
should be about 40 miles per hours, and definitely not 10,000 miles per
hour.
MonMotha wrote:
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
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