From: Akira Fujita <a-fujita@rs.jp.nec.com>
Subject: Re: [RFC][PATCH 0/3] ext4: online defrag (ver 1.0)
Date: Tue, 03 Feb 2009 17:00:26 +0900
Message-ID: <4987F99A.1050208@rs.jp.nec.com>
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Cc: Theodore Tso <tytso@mit.edu>, linux-ext4@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
To: Chris Mason <chris.mason@oracle.com>
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Hi Chris,

Chris Mason wrote:
> On Fri, 2009-01-30 at 15:11 +0900, Akira Fujita wrote:
>> Hi,
>>
>> I have rewritten ext4 online defrag patches based on the comments from Ted.
>> In the new defrag, create donor inode in the user space instead of kernel space,
>> and then allocate contiguous blocks to it with fallocate().
>> In kernel space, exchange the blocks between target inode and donor inode,
>> and then copy the file data of target inode to donor inode every 64MB.
>> The EXT4_IOC_DEFRAG ioctl becomes simpler than the old one,
>> so it may be useful for other purposes.
>>
> 
> One thing you'll want to handle is swap files.  The swap code uses the
> bmap ioctl to make a mapping of extents in the files, and expects that
> mapping not to change.  So, defragging a swap file will lead to some
> serious problems.
> 
> Btrfs is currently getting around this by dropping bmap support, so
> swapfiles on btrfs won't work at all.  A real long term solution is
> required ;)
> 
> For ext4 you should be able to just detect swapfile and disallow the
> defrag on it.
> 

Thank you for teaching.   ;)
I'll add the swapfile checks to command and kernel.
If target file is swapfile, ext4 online defrag returns an error
without doing defrag in the next version.

Regards,
Akira Fujita