linux-ext4 - Re: fsync stuck at jbd2_log_wait

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Date:	Wed, 12 Aug 2015 14:37:50 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Roy Yang <bsdnet@...look.com>
Cc:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: fsync stuck at jbd2_log_wait_commit on NVMe devices

Hi Roy,

My suggestion is to collect information from the jbd2_run_stats and
jbd2_checkpoint_stats tracepoints.

cd /sys/kernel/debug/tracing
echo 1 > events/jbd2/jbd2_run_stats/enable
echo 1 > events/jbd2/jbd2_checkpoint_stats/enable
cat trace_pipe > /tmp/traces &
tail -f /tmp/traces

The jbd2_handle_stats tracepoint can be informative, but it's also far
more voluminous.

That will give us a hint where things are getting bottlenecked.

What sort of workload is your application doing?  Is it just primarily
doing random writes into a preallocated file?  Is it creating or
deleting files?  Extending or truncating files?   etc.

	 	 	      		 - Ted
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