[geeks] Flash drive questions

[Mike Meredith]

Hi

I probably shouldn't be replying now as I've had rather too much of a
very nice 1966 vintage port ...

On Sun, 6 Aug 2006 15:33:27 -0400, James Fogg wrote:
> > 1: fixed disks are *slow* and they're getting slower relative to the
> >    rest of the computer's speed.
> 
> Fixed disks are rather fast compared to flash devices. And flash

They're still rather slow compared with computers ... being generous the
fastest fixed disk probably transfers data at less than 256MBps whereas
a system interconnect such as a Craylink works at 1.6GBps (I'm
deliberately picking something *old*).

This most definitely *is* a problem at least on database servers, and if
the fix is cheap enough I'd be happy to have my workstation fixed as
well.

> devices can only be written/erased a certain number of times before
> they die.

Certainly, but the problem is over-rated. Firstly, flash devices spread
the write load across the whole device when they can. Secondly modern
flash devices are reliable up to around a million write cycles.

> > 2: Moving parts (fixed disks) are inherently slower than solid state
> >    parts (flash). I haven't spent much time researching, but
> 
> Fixed disks aren't as fast as system memory, true, but are faster than
> flash memory. As for read speed, USB devices cannot read as fast as a
> fixed disk due to the faster interface for disks.

Firstly flash memory isn't *that* slow ... I've seen a comparison
benchmark of a flash-based laptop hd replacement that at worst is half
the speed of fixed disks. It's also getting quicker faster than fixed
disks, and that's just the consumer end of the market. 

Don't be distracted by the consumer market ... their products are
dominated by the requirement to be cheap and portable. Give an engineer
a flash chip which can write at 128Kbps and a brief that says "don't
worry about cost", you should end up with something dramatically quicker
:-

	512KBytes (disk block size) =
	512 * 8 * 1024 bits = 
	4194304 bits or chips (unrealistic I know) =
	4194304 * 128 Kbps =
	512 Gbps

My calculations could be off considering the port, but you can do writes
in parallel.

Just in case I'm wildly off base, Samsung recently announced a
flash-based laptop which reads at 53MB/s and writes at 28MB/s which
doesn't seem so slow to me :-

http://www.samsung.com/PressCenter/PressRelease/PressRelease.asp?seq=20060523_0000257520

> 
> > 3: Because of the write limits, you would be very unlikely to use
> >    flash-based storage for "swap space"; a cache for most frequently
> >    paged in stuff makes more sense (mostly reads).
> 
> Not true swap, but it would end up doing the job of swap space. And if

We haven't had true swap since PDP11's (well that's a little white lie
but you get the idea).

> you think you want to keep the most reusable info in the USB
> faux-system memory, you'd have to re-write all your OS's to do that,
> especially since you'd be talking about the highest memory addresses.

Not necessarily although that would be the best method. A symlink could
do much of the work (/lib/lib.so -> /media/superfastflash/libc.so).