[Bug 197516] TRIM on gmirror is slow and results in inresponsive system

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=197516

            Bug ID: 197516
           Summary: TRIM on gmirror is slow and results in inresponsive
                    system
           Product: Base System
           Version: 10.1-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: rs at bytecamp.net

Created attachment 152841
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=152841&action=edit
Test results

After deleting a (fairly) large file on an ssd based gmirror, the system
becomes inresponsive for a certain period of time. This seems to be caused by
the calls to BIO_DELETE on the mirror, which can be monitored with gstat -d.
Doing exactly the same on an unmirrored UFS leaves the system unimpacted, as
the DELETE operation takes only a fraction of time there.

I've also tried tuning the behaviour of the mirror with the sysctl
kern.geom.dev.delete_max_sectors. Setting it to 0 oder greater values than the
default of 262144 does not significantly affect the performance.

A script which I used to test with is attached, also the results reformatted in
a better readable way.

The system is non productive and can be used to figure out what is going wrong.
Any help is appreciated.

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