[luau] No hard drive, only compact flash card

Hawaii Linux Institute wp at HawaiiLinux.us
Sun Jul 6 22:46:01 PDT 2003


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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