linux-ext4 - Re: sync causing a flush with no data

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Message-ID: <87le7xbp68.fsf@vps.thesusis.net>
Date: Tue, 06 Feb 2024 10:14:23 -0500
From: Phillip Susi <phill@...susis.net>
To: linux-ext4@...r.kernel.org
Subject: Re: sync causing a flush with no data

Phillip Susi <phill@...susis.net> writes:

> It looks like ext4_sync_fs() calls jbd2_trans_will_send_data_barrier(),
> which returns 0, causing a call to blkdev_issue_flush().  Shouldn't this
> return a 1 if the transaction is empty?

Tracing a little more, it appears that
jbd2_trans_will_send_data_barrier() is returning 0 because the
transaction is already committed.  How can this be?  If a transaction
has been committed, shouldn't a new one be opened?  If not, and the
transaction indeed has already been committed, then why is that a reason
for the fs to issue another barrier?  If it has already been committed,
then the barrier should already have been issued shouldn't it?