[Nottingham] FLASH vs HDD

[Duncan John Fyfe]

On Tue, 2007-10-30 at 23:35 +0000, Martin wrote:
> OK Folks,
> 
> Continuing my deliberations for an 'ultimate PC' :-)
> 
> ... Next question (not that I've finalised the earlier questions yet...)
> 
> Will I gain much of a speedup by using a USB flash drive for storing the
> OS and applications?

You improve seek times (realistically milliseconds -> microseconds,
possibly better) and consequently the seek limit.  The write speed of
USB flash drives can suck compared to HDD though so buyer beware (see
below).

Why USB ?  
Would you consider CF->IDE or a solid state hard drive (You can get a
solid state 4GB PATA device for ~ 60 GBP.) ?

> 
> I've seen a 4GB pendrive advertised at "x150" speed. What is that x150 of?!
> 

It is relative to a x1 CD player/drive.  
You see this sort of thing referred to in camera mags along the lines of
"You should really use xN flash memory if you want F frames per second
video recording".  It is w*nk word for write speed to be used by
marketing people and people who think their flash device (trench coat ?)
is better "because it goes up to 11."  

> Anyone tried this?
> 
Not personally but I know solid state devices have been used in the HPC
sector where HDD seek limited access patterns are a problem.

> And what is the best fs for flash?
> 
I tend to use vfat and ext2 for portability on USB flash sticks.
I haven't used either but jffs2 or possibly logsf
(http://www.logfs.org/logfs/) are intended as flash based filesystems.

Are you thinking flash or flash as hidden by the controller ?
Some devices, especially modern USB sticks don't give you access to the
underlying flash chips but do provide on-stick wear levelling.  In such
cases it is probably best to avoid mixing on-stick and filesystem  wear
levelling (cf jffs2).

> 
> Or am I wasting my time?...
> 

It depends on what you put on the disk and the rest of your system and
your usage pattern....  Remember, this is disk not CPU addressable
memory so you won't be able to use execute in place (xip).  If it was
your system disk (/ , /usr etc)  then I can imagine an improved boot
time and application start time but once most of the significant
libraries are cached in memory (assuming enough - and being the ultimate
PC it must have :) the benefits will diminish.

You won't be able to buy enough to replace the 1+ TB disk array you
are trying to configure so you will probably see the greatest benefits
from using it for:
swap space;
/tmp; 
as fast work space and writing your apps (if any) to use it. Something
like:
	copy working set from HDD to flash
	work off flash copy
	copy results back to HDD


NB. If you are not using all of the memory on your graphics card you can
hive off the unused portion, configure it as a memory device and put
some high priority swap space on it.

> Any comments?

It might be better to turn the question on its head.
What problem(s) are you having that might be solved by using a solid
state disk ?

Have fun,
Duncan