linux-ext4 - O_DIRECT and delayed allocation question

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Message-ID: <6601abe90906181253u657e67a7u67025774cc53bc4f@mail.gmail.com>
Date:	Thu, 18 Jun 2009 12:53:17 -0700
From:	Curt Wohlgemuth <curtw@...gle.com>
To:	ext4 development <linux-ext4@...r.kernel.org>
Subject: O_DIRECT and delayed allocation question

This might be a question with an obvious answer, but I'd like
verification one way or the other.

Does the use of O_DIRECT essentially disable delayed allocation for a
given file?

My simple tests show a larger degree of block fragmentation for files
written using O_DIRECT than without, and on its face, this makes sense
to me.  This fragmentation can be removed by using fallocate() on a
file before extending it with writes.

(Strictly speaking, I guess the use of O_DIRECT wouldn't "disable"
delayed allocation, since the blocks are allocated at the "normal"
time -- when going to disk.  But effectively there would be a lot less
block grouping available to build large extents if each write goes to
disk immediately, instead of going through the page cache.)

Thanks,
Curt
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