Hi,
I did some benchmarks to check the maximum transfer rate of a USB-to-SCSI
converter. The converter is USB 1.1, so limited to the 12Mbps full speed
rate. The performance when connected to a USB 3.0 port is significantly
worse than when connected to a USB 2.0 port, about 26.5% slower (0.63MB/sec
vs 0.86MB/sec).
The USB 3.0 port is provided by an ExpressCard which has a Renesas
controller. It doesn't seem to be defective because I can get 138MB/sec
from a USB 3.0 hard disk. lspci shows
05:00.0 USB Controller: Renesas Technology Corp. Device 0015 (rev 02)
I didn't test with an unmodified mainline kernel, but the issue is present
with both Ubuntu kernel 3.0 and Debian 3.14 (booted from Kali Linux live
DVD).
I used sg_rbuf from the sg3-utils package. That issues READ BUFFER commands
instead of reading data from disk, so the result should be closer to the
maximum achievable. The USB-SCSI converter was a Newer Technology uSCSI
connected to an HP 2600fx MO drive.
In a USB 2.0 port:
# sg_rbuf --buffer=524288 -v -t -q /dev/sg3
Read buffer cdb: 3c 03 00 00 00 00 00 00 04 00
READ BUFFER reports: buffer capacity=983040, offset boundary=0
time to read data from buffer was 242.800241 secs, 0.86 MB/sec
Read 200 MiB (actual: 209715200 bytes), buffer size=512 KiB (524288 bytes)
# sg_rbuf --buffer=524288 -v -t -q /dev/sg3
Read buffer cdb: 3c 03 00 00 00 00 00 00 04 00
READ BUFFER reports: buffer capacity=983040, offset boundary=0
time to read data from buffer was 242.800230 secs, 0.86 MB/sec
Read 200 MiB (actual: 209715200 bytes), buffer size=512 KiB (524288 bytes)
In a USB 3.0 port:
# sg_rbuf --buffer=524288 -v -t -q /dev/sg3
Read buffer cdb: 3c 03 00 00 00 00 00 00 04 00
READ BUFFER reports: buffer capacity=983040, offset boundary=0
time to read data from buffer was 330.311337 secs, 0.63 MB/sec
Read 200 MiB (actual: 209715200 bytes), buffer size=512 KiB (524288 bytes)
# sg_rbuf --buffer=524288 -v -t -q /dev/sg3
Read buffer cdb: 3c 03 00 00 00 00 00 00 04 00
READ BUFFER reports: buffer capacity=983040, offset boundary=0
time to read data from buffer was 330.388556 secs, 0.63 MB/sec
Read 200 MiB (actual: 209715200 bytes), buffer size=512 KiB (524288 bytes)
By comparison, I checked the transfer rate in native Windows (Vista) with
the WinSAT program. That reported 0.66MB/sec for both USB 2.0 and 3.0
ports. (WinSAT reads data from disk, probably in smaller chunks than 512KB.
There's no sg_rbuf executable in the Windows sg3_utils archive.)
Any ideas what the reason for the discrepancy might be? Can anyone else
reproduce it? I guess USB 1.1-only devices aren't that common nowadays.
Mark
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