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

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Message-ID: <20091008114851.GA22610@skywalker.linux.vnet.ibm.com>
Date:	Thu, 8 Oct 2009 17:18:52 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: Where all does preallocated/extra space hide?

On Tue, Sep 29, 2009 at 11:47:19AM -0500, 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.  :)
> 

preallocation space is discarded by default if we fail a block allocation
ext4_mb_discard_preallocations does that. What might be happening is the
extra meta data blocks that we reserve for making sure we will be able
to properly insert the new extent on block allocation.  I guess we should
force a data allocation when we fail with ENOSPC in ext4_da_writepages
We currently force a journal commit so that the we claim back the blocks
from deleted files. But we can also force block allocation for delayed
allocated inodes so that we free some of the extra meta data we reserved

-aneesh
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