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

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Date:	Fri, 9 Oct 2009 10:57:04 +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 Thu, Oct 08, 2009 at 10:56:25AM -0500, Eric Sandeen wrote:
> Aneesh Kumar K.V wrote:
> > 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
> 
> Yep, I should have followed up, I narrowed it down to just that - the
> worst-case metadata blocks - 2 metadata blocks for a 20-byte write into
> an empty file.  :)
> 
> I'm working on an inode walker to push out delalloc files on enospc.


Should we do an inode walker ? I guess we should be doing something
similar to balance_dirty_pages. That will kick in the flusher threads
which inturn will force the block allocation of dirty inodes. 

-aneesh 
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