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Posted by Mark F on April 22, 2008, 6:25 pm
Please log in for more thread options On Tue, 22 Apr 2008 07:58:18 +0100, "M.I.5¾"
>
> >I am having issues with compact flash card that I got. I is writing very
> > slow. It's a 32gb, 233x card. The read speed is unbelievable, but the
> > write
> > speed is less then 1mb/s. What's wrong? It supposed to get at least
> > 20mb/s.
> >
>
> The first thing to note is that the write speed is always lower than the
> read speed. This is due to the way FLASH memory actually does its writing.
> Whenever you write anything, even if its only one byte, an entire block of
> the memory is copied in to a buffer, the data altered as required, the
> target block erased, the buffer then written to it and the block journal
> updated to show where the block actually is (which requires another block
> read, erase and write cycle). As you can appreciate this takes much longer
> than just a simple read with the last six parts taking the longest. The
> larger sizes of memory are generally made up from memory cells which are
> much faster than their earlier counterparts, but a problem arises if the
> card reader doesn't fully understand the architecture that some of the
> larger sizes use or indeed their speed, and can introduce a bottle neck in
> the data flow. This will certainly be the case if you FLASH memory reader
> is more than a year or two old.
What you say is true, so the issue is how can the original poster get
close to the published speed specification. The techniques I use
include:
. writing large files that are allocated in one step. If your program
doesn't allocate in one step, try sending the output to a hard disk
and then copy the full file to the flash memory. Speeds up some
backup programs by a factor of 100 or more since even though the
program has a good estimate of the total size needed it allocates
space in very small pieces.
. use VMware or some other virtual disk software to put container
files for a virtual disk on the flash memory key and write to
the flash memory key through the virtual disk interface.
>
> There is also a considerable amount of counterfeit memory out in the wild
> that although marked as some of the larger sizes and reports itself as that
> larger size, is in fact much much smaller (and much much slower). The
> larger sizes are prime targets as they command the premium prices.
>
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