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What's wrong with df?

To: Linux XFS Mailing List <linux-xfs@xxxxxxxxxxx>
Subject: What's wrong with df?
From: Federico Sevilla III <jijo@xxxxxxxxxxxxxxx>
Date: Mon, 14 May 2001 18:52:18 +0800 (PHT)
Sender: owner-linux-xfs@xxxxxxxxxxx
Hi everyone,

While migrating a ReiserFS partition to XFS, I noticed that df reported a
pretty significant space utilization difference between the two
filesystems. I transferred my files using two methods, and both had
identical results.

First I used tar from /xfs: "tar -cl /reiserfs | tar xv", then I used cp:
"cp -a /reiserfs /xfs". Here is the output when I ran "df -h" after each
file transfer operation:

Filesystem      Size    Used    Avail   Use%    Mounted on
/dev/hda5       2.0G    692M    1.4G    33%     /reiserfs
/dev/hda3       952M    746M    206M    78%     /xfs

If this is accurate (although it doesn't seem accurate as I will later
elaborate), then XFS uses 52MB more than ReiserFS does for a relatively
small partition (this is actually my root partition, and I only exclude a
small boot and the home partition).

I decided to check using du by running "du -csm" and found that ReiserFS
uses 734MB and XFS uses 733MB. This looks more accurate. Would anyone know
what's wrong with df? It's a much handier tool to use than du, but if it's
not accurate then it doesn't look like a good tool to use, handy or not.

Thanks in advance for your comments and suggestions.

 --> Jijo

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Linux, MS-DOS, and Windows NT ...
... also known as the Good, the Bad, and the Ugly


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