linux-ext4 - Re: 2.6.33.1: ext4 disk free weirdness

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Date:	Mon, 29 Mar 2010 10:43:09 -0700
From:	Justin Maggard <jmaggard10@...il.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: 2.6.33.1: ext4 disk free weirdness

On Mon, Mar 29, 2010 at 8:01 AM, Eric Sandeen <sandeen@...hat.com> wrote:
> tytso@....edu wrote:
>> On Mon, Mar 29, 2010 at 09:48:50AM -0500, Eric Sandeen wrote:
>>> CaT wrote:
>>>> Using kernel 2.6.33.1. Am rsyncing from /data/mirror (a raid0 mount) to
>>>> /dev/sda1 (a usb HD). Both filesystems formatted as ext3 but mounted as
>>>> ext4. /mnt filesystem is only being added to and currently large files
>>>> (iso files 700MB-4.4GB) are being copied. When I remounted /mnt as ext3
>>>> I could not duplicate the issue and the amount of disk used stabilised
>>>> at 182G. The df results, whilst /mnt is mounted as ext4, are below:
>>> I would say this is likely speculative allocation due to delalloc
>>> (ext4 has to reserve worst-case metadata amounts to prepare for
>>> delalloc writeback).  When the file data actually gets written out,
>>> the worst-case reservation is freed up again.  We also flush and
>>> switch to nodelalloc when the filesystem is pretty close to full.
>>> see ext4_nonda_switch() for example.
>>>
>>> This looks pretty severe though, more than I would expect
>>> from that behavior.
>>
>> This is fixed in commit d330a5bef, which which got merged post
>> 2.6.34-rc2, and which we need to get into 2.6.33 stable.  If you
>> cherry-pick that commit, I think you'll see that it fixes this
>> problem.
>
> Oh, right, spaced that one.  Thanks for the reminder Ted.  :)
>
> And from now on ext3-mounted-as-ext4 is special-cased so you won't get
> delalloc, and you won't get this behavior at all (nor will you get
> any delalloc benefits, FWIW).
>

Out of curiosity, when you say ext3-mounted-as-ext4, are you referring
to filesystems without ext4 features mounted -t ext4, or filesystems
mounted -t ext3 when CONFIG_EXT4_USE_FOR_EXT23 is defined?

-Justin
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