"Disk" bandwidth

Stefan Monnier monnier at iro.umontreal.ca
Wed Nov 26 17:46:09 CET 2008

I recently noticed the following behavior:

   # hdparm -tT /dev/mtdblock6 /dev/mmcblk0
    Timing cached reads:    48 MB in  2.01 seconds =  23.88 MB/sec
    Timing buffered disk reads:   10 MB in  3.05 seconds =   3.28 MB/sec
    Timing cached reads:     2 MB in  2.28 seconds = 899.42 kB/sec
    Timing buffered disk reads:    4 MB in  4.42 seconds = 927.64 kB/sec

The bandwidth returned for mtdblock6 actually varies a good bit (cached
bandwidth can go from 5MB/s to 40MB/s and disk reads can go from
1.8MB/s to 3.6MB/s), but the bandwidth for the uSD card is pretty much
always around the one reported above, i.e. very low.

The kernel used is the "stable" one from several weeks back and I heard
that the NAND access has been improved in the mean time, which is great.
But what about uSD access?  Is that really the best we can hope for?

And it seems that the uSD access is not cached.  Why is that?  Can it
be changed?

        Stefan "Running Debian in NAND"

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