During a receive operation zvol_create_minors_impl() can wait
needlessly for the prefetch thread because both share the same tasks
queue. This results in hung tasks:
<3>INFO: task z_zvol:5541 blocked for more than 120 seconds.
<3> Tainted: P O 3.16.0-4-amd64
<3>"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
The first z_zvol:5541 (zvol_task_cb) is waiting for the long running
traverse_prefetch_thread:260
This change adds a dedicated, per-pool, prefetch taskq to prevent the
traverse code from monopolizing the global (and limited) system_taskq by
inappropriately scheduling long running tasks on it.
Reviewed-by: Albert Lee <trisk@forkgnu.org> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6330
Closes #6890
Closes #7343