Ulrich --- if you have time to try to do more tests, may I gently suggest that you start a new Launchpad bug?    Drop a pointer from in this launchpad bug number to the new one, but let's do all of the experimentation on a new launchpad bug.   There are a couple of reasons for this:

(1)  Launchpad has horrific scalability problems, and it's painfully slow to find new entries on this Launchpad entry, since it has so many comments already.

(2)  It's not clear everyone on this bug is sufferring from the same bug.   They may have similar symptoms, but that doesn't mean it is the same bug.   It's actually **highly** confusing to have many people glomming on with a "me, too!!" comment, giving a very tiny amount of information, but not actually giving a full description of what they are seeing.   In fact, the tiny amounts of data may be horribly misleading, because they may be seeing a different bug.

(3) As a result, many kernel developers on LKML don't spend a lot of time on Launchpad bugs; the bug reporting is ___so___ ___bad___ that it's just too frustrating and time-consuming for them.   And unfortunately, Canonical has a fairly limited kernel team, and it takes them a lot of time try to synthesize a good bug report from a lousy one.  In fact, someone recently posted this bug report on LKML, and what it mostly got was a vent from someone complaining how useless most Ubuntu bugs were.  (Well, it also got me interested enough to actually take a look, but I've gotten inured to how lousy Ubuntu bug reporting infrastructure is for large bugs, as well as mostly uninformative bug reports.)

So if we do have one or more people who have the time and energy to do some real bug reporting (which takes real work), may suggest that each person open a separate bug, and each bug, give (a) full details about what kernel version, including whether it is a Ubuntu kernel or a mainline kernel, (b) full details about your hardware version, including which disks are hooked up which way (i.e., eSATA, USB, SATA, PATA, etc.), and (c) exhaustive details about how you ran your test case, whether or not is repeatable, etc.   Also note that it is an instance of Launchpad bug #197762, and that anybody else who wants to glom on with a "me, too" is kindly asked to put that remark in the #197762 cesspit^H^H^H^H^H^H^H^Htop-level bug, and not in the new bug report, which we can hopefully keep specific to your situation.

I would again suggest that you try doing multiple tests:

1)  Copying one file from one disk to another
2)  Copying from /dev/zero to file on the disk
3)  Copying from /dev/zero to the raw disk device (yes, this will blow away your filesystem; hopefully you have scratch devices)

The ideal bug reporter will have multiple devices that can be attached via different hardware interfaces (i.e., eSATA, USB, maybe a spare SATA port), so we can try to rule out hardware attachment problems.   

If someone is willing to do all of that, I'm more than willing to try to drill down as far as I can; although if it turns out to be a USB problem, I'll need to call in help from a USB developer ---- but with a well constructed bug report, that shouldn't be a problem; I should be able to overcome LKML prejudices about lousy Launchpad bugs ("no really, this one is different; we've kept it clean and professional, and there's plenty of information --- and we have a technically cluefull user who is willing to work with you and run experiments until the problem can be solved")

More generally, maybe someone with the time can start a Wiki page on debugging filesystem performance problems, maybe on the Ubuntu wiki somewhere, or on ext4.wiki.kernel.org.    There are plenty of tools like that people can use for debugging performance problems, beyond just "dstat".   Other tools include blktrace, sar, iostat, vmstat, and many others.   Launchpad is really a lousy place to have these discussions; a wiki or a forum topic is really much better.