@Andrea, your report is closely related to some other reports including
#343371 and #131094. See my comment and request at
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/131094/comments/235.
In your case, there are (potentially) two issues. First, there is the
latency under I/O load and, second, there may be still an issue with cfq
with the pen drive. We would like you to do the testing described in
the comment link first and if there is still a pen drive problem, we can
address that next.
The I/O schedulers have been focused primarily on HDDs. The typical
drive has a shared filesystem on it and, being mechanical, has
rotational and seek latencies that must be compensated for. The CFQ
scheduler does a pretty good job of fair and sustained throughput in
that environment. On the other hand, SSDs and pen drives, actually any
flash based device, have a different set of constraints. There is no
seek/rotational latency at all but write performance can be pretty bad,
especially for small, random writes. This is because the controller in
the drive must read a whole block (usually 64k+), copy the small write
into it, erase the block, and then, finally, write the block back, most
often in a different location to "wear level" the device. SSDs are
still evolving but current parts do not perform well with short writes.
They are getting better but they are not there yet. This is a work in
progress. Pen drives do not get even this much attention given that
they are a single user/task, offline storage device.
Using NOOP scheduling does seem to work better than CFQ for these
devices but from your comment, the dd is only doing 512 byte "records".
Try using a larger block size (> 1MB). That seems to work well enough
to make scheduling a non-issue.
I have changed this to incomplete pending your test results. I may mark
it as a duplicate based on those results.
Thanks
** Changed in: linux (Ubuntu)
Status: Triaged => Incomplete
** Changed in: linux (Ubuntu)
Assignee: (unassigned) => Jim Lieb (lieb)
--
elevator=cfq (default) cause starvation
https://bugs.launchpad.net/bugs/381300
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