USB 2 & umass

Alan Batie alan at
Sat Aug 30 15:08:25 PDT 2003

OK, the point of this whole exercise was to find out if what I'd heard
about USB 2 being half duplex and thus a bad idea for RW mass storage
was true.  It looks like it, but on the other hand, it looks like I'm
only running at USB 1 speeds too:

/mnt is the USB drive, which has a fresh FreeBSD installation on it for
something else, so I'm playing in /mnt/tmp for the USB side, and /tmp for
the native IDE side:

<> [72] $ cat ../kernel ../kernel > k
<> [73] $ time cat k k > k2
real    0m15.408s

<> [77] $ time cat k2 k2 > k4
real    0m32.561s

<> [78] $ lf -l k4
-rw-r--r--  1 alan  wheel  32978776 Aug 30 14:42 k4, about 1MB/s

<> [81] $ time cat k4 > /tmp/k4
real    0m2.946s

...on the other hand, unidirectional USB->IDE gets 10MB/s
(an old 6G Quantum Fireball, not a fast drive, but this does imply
that it's running faster than USB 1's 12Mb/s, which would be 1.5MB/s
absolute best case)

<> [82] $ cd /tmp
<> [83] $ time cat k4 k4 > k8
real    0m5.917s

...a little over 10MB/s

<> [85] $ time cat k8 > /mnt/tmp/k8
real    1m5.876s

...and back to 1MB/s IDE->USB (r/w'ing directory/inode info?)

<> [87] $ lf -l k8
-rw-r--r--  1 alan  wheel  65957552 Aug 30 14:43 k8

I also just noticed this tidbit in the boot messages, FWIW:

(da0:umass-sim0:0:0:0): READ(6)/WRITE(6) not supported, increasing minimum_cmd_size to 10.

On the other hand, this:

<> [95] $ time cat k8 > /dev/null
real    0m51.805s

...which, at about 1+MB/s implies the 10MB/s above was a cache effect,
and thus the question: why is this thing running at USB 1 speeds?

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