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

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Message-ID: <4ACF4DF3.3030605@redhat.com>
Date:	Fri, 09 Oct 2009 09:51:31 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
CC:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: Where all does preallocated/extra space hide?

Aneesh Kumar K.V wrote:
> 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 

it'll need to be synchronous to avoid a spurious enospc I think ...
plus, we only want to flush delalloc inodes; flushing everythign would
be needlessly expensive I think... need to think about the right way to
do this.

-Eric
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