USB I/O performance

[Bill Davidsen]

Patrick O'Callaghan wrote:
> Just a quick note to call people's attention to
> http://marc-abramowitz.com/archives/2007/02/17/getting-good-performance-out-of-usb-hard-drives-in-linux/. This is a couple of years old but it worked like a charm for me.
> 
> Briefly, there's a kernel parameter
> called /sys/block/sd[a,b,...]/device/max_sectors (for USB drives sda,
> sdb etc.). This specifies the maximum size of a disk I/O operation for
> USB storage devices in units of 512 bytes, the default value being 240,
> i.e. 120KB (see http://www.linux-usb.org/FAQ.html#i5). The max_sectors
> value can be changed doing "echo N > ..." as root, and can have a
> dramatic effect on write performance for USB devices such as pendrives.
> 
> I tested this by writing over 2GB to a fresh VFAT filesystem on a 4GB
> Kingston Data Traveller pendrive plugged into a USB2 port with the EHCI
> driver (as indicated by dmesg). With the default setting, this took
> nearly 90 minutes including a final sync to flush the buffers. Using a
> max_sectors value of 1024 -- the highest the system would accept -- the
> time was reduced to under 16 minutes, a better than 5 times speedup.
> YMMV of course, as different brands of pendrive can have very different
> performance characteristics.
> 
This has been discussed in other groups, and the issue seems to be that a write 
to flash time is described as a constant value plus a size dependent value, and 
that by doing fewer and larger writes the penalty of the constant is less, since 
it is incurred less often.

> Note that the value resets to the default when you unplug the drive, so
> you need to set it manually each time. I don't know if there's a way to
> do this automatically, or change the default value permanently.
> 
> Sorry of everyone already knew this, but I found it so useful I just had
> to share it :-)
> 
I found that it made essentially no difference in read speed (4300k/s vs 
4400k/s) so people should be sure they note that "write" is specified in the 
above 1st paragraph, don't overlook it!

Didn't seem to do much for write speed on a USB connected hard drive, so the 
significant benefit seems limited to write to flash. That's important, but for 
many uses there are far more reads than writes.

HINT:I wish it were a standard feature of install to a flash drive. Manually 
setting it during install is probably worth trying. Any FC11 developers reading 
this? HINT!!

-- 
Bill Davidsen <davidsen at tmr.com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot