linux-ext4 - Re: [RFC 00/11] DAX fsynx/msync support

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Date:	Mon, 02 Nov 2015 09:22:15 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	"Theodore Ts'o" <tytso@....edu>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Dan Williams <dan.j.williams@...el.com>,
	Ingo Molnar <mingo@...hat.com>, Jan Kara <jack@...e.com>,
	Jeff Layton <jlayton@...chiereds.net>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-nvdimm@...1.01.org, x86@...nel.org,
	xfs@....sgi.com, Andrew Morton <akpm@...ux-foundation.org>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>
Subject: Re: [RFC 00/11] DAX fsynx/msync support

Dave Chinner <david@...morbit.com> writes:

> Further, REQ_FLUSH/REQ_FUA are more than just "put the data on stable
> storage" commands. They are also IO barriers that affect scheduling
> of IOs in progress and in the request queues.  A REQ_FLUSH/REQ_FUA
> IO cannot be dispatched before all prior IO has been dispatched and
> drained from the request queue, and IO submitted after a queued
> REQ_FLUSH/REQ_FUA cannot be scheduled ahead of the queued
> REQ_FLUSH/REQ_FUA operation.
>
> IOWs, REQ_FUA/REQ_FLUSH not only guarantee data is on stable
> storage, they also guarantee the order of IO dispatch and
> completion when concurrent IO is in progress.

This hasn't been the case for several years, now.  It used to work that
way, and that was deemed a big performance problem.  Since file systems
already issued and waited for all I/O before sending down a barrier, we
decided to get rid of the I/O ordering pieces of barriers (and stop
calling them barriers).

See commit 28e7d184521 (block: drop barrier ordering by queue draining).

Cheers,
Jeff
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