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Subject: zpool create on md hangs
From: Curtis Villamizar <curtis@ipv6.occnc.com>
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The following shell program produces a hang.  Its reproducible (hangs
every time).

    #!sh

    set -e
    set -x

    truncate -s `expr 10 \* 1024 \* 1024 \* 1024` /image-file
    md_unit=`mdconfig -a -n -t vnode -f /image-file`
    echo "md device is /dev/md$md_unit"
    zpool create test md$md_unit

The zpool command hangs.  Kill or kill -9 has no effect.  All
filesystems are unaffected but any other zpool or zfs command will
hang and be unkillable.  A reboot is needed.

This is running on:

   FreeBSD 10.0-STABLE (GENERIC) #0 r270645: Wed Aug 27 00:54:29 EDT 2014

When I get a chance, I will try again with a 10.1 RC3 kernel I
recently built.  If this still doesn't work, I'll build an r11 kernel
since the code differs from 10.1, not having the svm code merged in.
I'm asking before poking around further in case anyone has insights
into why this might happen.

BTW- The reason to create a zfs filesystem on an vnode type md is to
create an image that can run under bhyve using a zfs root fs.  This
works quite nicely for combinations geom types (gmirror, gstripe,
gjournal, gcache) but zpool hangs when trying this with zfs.

Curtis

ps- please keep me on the Cc as I'm not subscribed to freebsd-fs.