linux-ext4 - Re: Where all does preallocated/extra space hide?

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Message-ID: <4AC4DF13.10704@redhat.com>
Date:	Thu, 01 Oct 2009 11:55:47 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: Where all does preallocated/extra space hide?

Eric Sandeen wrote:
> I was running some of the xfstests enospc tests on ext4, and they were
> failing; in one case, manymanymany small files are made to fill up a
> 100M filesystem.  ext4 stops quite early with -ENOSPC, but after a bit,
> (or after a "sync") we get 40MB free again.  So 40% of the fs space is
> hidden somewhere in preallocation...
> 
> I tried calling out to discard group prealloc but that's only a few
> blocks.  I'll go trace through the sync paths to see what all gets
> released, but if anyone knows offhand where the rest of that space is
> hiding, please give me a shout.  :)
> 
> Thanks,
> -Eric

Possibly related; on a 1G filesystem, doing this:

#!/bin/bash

xfs_io -F -f -d -c 'pwrite -b 64k 0 512m' /mnt/test/io_test
rm /mnt/test/io_test

in a "while true" loop spews ENOSPC.  (it's a direct IO 512m write in 
64k chunks).  Buffered IO seems fine ...

-Eric
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