linux-ext4 - Re: defragmentation of boot related files

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Date:	Thu, 26 Aug 2010 14:10:08 -0400
From:	Greg Freemyer <greg.freemyer@...il.com>
To:	Kazuya Mio <k-mio@...jp.nec.com>
Cc:	andreas@...-net.de, linux-ext4@...r.kernel.org
Subject: Re: defragmentation of boot related files

2010/8/26 Kazuya Mio <k-mio@...jp.nec.com>:
> Hi Andreas,
> Thanks for the comments.
>
> 2010/08/25 21:00, andreas@...-net.de wrote:
>> Is there a reason why the offset of the original file and the donor file
>> must be the same?
>
> e4defrag creates a donor file whose size is the same of the original file by
> fallocate. There is a possibility that the original file will be corrupted
> after moving an extent if the offset of the original file and the donor file
> are different. So they are checked in the kernel space, but it may be
> unnecessary from the point of view of the ioctl.
>
>> As i can see the patch for relevant file defragmentation in e4defrag
>> supports only directories. May it be possible to select any desired file?
>
> That's interesting. I came up with the new interface of e4defrag -r.
> What do you think the following implementation idea?
>
> Usage: e4defrag -r directory...| device...
>       e4defrag -r base_file move_file...     <--- new
>
> 1. Defrag base_file to reduce fragmentation of extents (call EXT4_IOC_MOVE_EXT)
> 2. Preallocate physical blocks near the data blocks of base_file
> 3. Move move_file's extents to the blocks that are allocated by (2).
> 4. Repeat (2) and (3) for all files specified as move_file
>
> Regards,
> Kazuya Mio

I suspect the original idea would work better because it is more
likely to pack the libs / files into perfectly contiguous block
ranges.

I too have never understood why the donor offsets have to match the
original offsets, although I had previously assumed the issue could be
worked around via making the donor file sparse and only have the block
range of interest allocated.

This use case is a specific example of where it would be beneficial to
eliminate that artificial limitation.

Greg
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