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Gmane
From: Chris Mason <chris.mason <at> oracle.com>
Subject: Re: [RFC][PATCH 0/3] ext4: online defrag (ver 1.0)
Newsgroups: gmane.comp.file-systems.ext4
Date: Friday 30th January 2009 20:15:18 UTC (over 8 years ago)
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.

-chris


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