linux-ext4 - Minimizing fragmentation in ext4, fallocate not enough?

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Message-ID: <4C9D3CD6.9080000@mozilla.com>
Date:	Fri, 24 Sep 2010 17:05:42 -0700
From:	Taras Glek <tglek@...illa.com>
To:	linux-ext4@...r.kernel.org
Subject: Minimizing fragmentation in ext4, fallocate not enough?

Hi,
I noticed that several random IO-heavy Firefox files got fragmented 
easily. Our cache suffers most.
The cache works by creating a flat file and storing fixed-size entries 
in it. I though if I fallocate() the file first, then all of the writes 
within the allocated area would not cause additional fragmentation.

This doesn't seem to completely cure fragmentation with ext4 in 2.6.33. 
If I allocate a 4mb file, it gets more and more fragmented over time. 
fallocate() does reduce fragmentation, but not as much as I expected.

I assumed that writing to an fallocate()ed area will not cause 
additional fragmentation. Is my assumption incorrect?

Thanks,
Taras

ps. I'm using filefrag for measuring fragmentation.

ps2. Does running filefrag on a directory mean anything in ext4?
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