linux-ext4 - Re: fdatasync/barriers (was : [Bug 421482] Firefox 3 uses fsync excessively)

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Message-ID: <OF96824E74.387F8AD7-ON88257458.005D1BC3-88257458.005E2EC4@us.ibm.com>
Date:	Thu, 29 May 2008 10:08:44 -0700
From:	Bryan Henderson <hbryan@...ibm.com>
To:	Jamie Lokier <jamie@...reable.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jörn Engel <joern@...fs.org>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Theodore Tso <tytso@....EDU>
Subject: Re: fdatasync/barriers (was : [Bug 421482] Firefox 3 uses fsync excessively)

> Here's a thought for someone implementing fdatasync().  If a database
> uses O_DIRECT writes (typically with aio), then wants data which it's
> written to be committed to the hard disk platter, and the filesystem
> is mounted "barrier=1" - should it call fdatasync()?  Should that emit
> the barrier?  If another application uses normal (not O_DIRECT)
> writes, and then _is delayed_ so long that kernel writeback occurs and
> all cache is clean, and then calls fdatasync(), should that call emit
> a barrier in that case?  (Answers imho: yes and yes).

I don't get it.  What would be the value of emitting the barrier?

--
Bryan Henderson                     IBM Almaden Research Center
San Jose CA                         Filesystems

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