[Bug 244356] Writing to a USB 3.0 stick is very slow

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=244356

--- Comment #87 from Gary Jennejohn <gljennjohn at gmail.com> ---
(In reply to Olivier Certner from comment #86)
I only used dd for my testing.  A file system adds lots of overhead and does
not give a clear picture of the raw performance.
I used a Kingston DT Rubber 3.0ge 16GB on both FreeBSD and Linux.  I used it
because it's easy to fill up such a small stick.
Unfortunately, the FreeBSD XHCI driver does not correctly attach the stick
(probably a bug) whereas USB2 has no problems with it.  Under Linux XHCI is
happy to use it.
This is a rather old stick (9 or 10 years) and Kingston claims that it can only
write at a maximum of 30MB/s.  A test report I saw showed that the stick can
actually get up to about 40MB/s.
For the dd test I wrote two different approximately 16GB files to the stick one
after the other.  I tried both bs=128k and bs=256k, but that had no effect on
the transfer rates under FreeBSD or Linux.
In any case, whether the stick was filled with 0xFF or all sectors were already
dirty didn't change the write transfer rates.  Under FreeBSD (USB2) I saw about
33MB/s and under Linux (USB3) I saw about 40MB/s for each of the files.
I also used the 128GB (USB3) stick with same dd tests under FreeBSD and also
saw on the order of 40MB/s in that test.  This stick probably was not full.
So, I guess the poor write performance isn't caused by the contoller in the
stick.  It's probably more likely that the filesystem overhead is the culprit,
especially with UFS when the stick is getting full or when it's writing small
files into fragments.

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