"Disk" bandwidth

[Andy Green]

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Somebody in the thread at some point said:

| # hdparm -tT /dev/sdg
|
|    /dev/sdg:
|     Timing cached reads:   1286 MB in  2.00 seconds = 643.14 MB/sec
|     Timing buffered disk reads:   26 MB in  3.13 seconds =   8.32 MB/sec
|    #
|
| where /dev/sdg is a USB cardreader reading a uSD card.  Obviously,
| USB-2.0 is not able to provide 640MB/s so the cache is elsewhere.

I eyeballed the hdparm sources, the difference between these guys is
indeed meant to be the Linux block cache.  In the cached read case it
keeps seeking to 0 and doing a big read, in the noncached "buffered"
case it keeps on reading new data and not seeking to zero.

Hdparm does also concern itself with the drive cache but not directly in
this test.

If /dev/mmcblk0* was mounted at this time, maybe confirm the mount
options used, eg, sync... I don't even know how to opt out of the buffer
cache if we wanted to :-/

- -Andy
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