linux-ext4 - Re: [PATCH 01/11 RESEND] libe2p: Add new function get_fragment

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Date:	Sat, 25 Jun 2011 22:16:54 -0400
From:	Greg Freemyer <greg.freemyer@...il.com>
To:	Kazuya Mio <k-mio@...jp.nec.com>
Cc:	Andreas Dilger <aedilger@...il.com>,
	Eric Sandeen <sandeen@...hat.com>, "Ted Ts'o" <tytso@....edu>,
	ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 01/11 RESEND] libe2p: Add new function get_fragment_score()

On Fri, Jun 24, 2011 at 4:28 AM, Kazuya Mio <k-mio@...jp.nec.com> wrote:
> Hi Greg,
> I'm sorry for the late reply.
>
> 2011/06/23 20:16, Greg Freemyer wrote:
>>
>> For a sparse file, can you explain why you treat the head and tail
>> extents of a block group differently?
>
> Could you tell me what "block group" you said means?

Kazuya,

Sorry if I've got the terminology wrong.

I'm talking about sparse files.

Where you might have:

block_group1 - hole - block_group2 - hole - block_group3.

Block_group2 has a head and a tail extent.  In my mind, from a
performance perspective, they are symmetric.  Meaning that having a
small extent at the beginning is no better and no worse than having a
small extent at the end.

> If "block group" means the ext4 block group, I will treat the head and tail
> extents of a block group the same way.
>
> And if "block group" means the chunk of the extents whose offset is
> continued,
> I will treat only the tail extents as a special case.
>
> # filefrag -v /mnt/mp1/file
> Filesystem type is: ef53
> File size of /mnt/mp1/file is 285212672 (69632 blocks, blocksize 4096)
>  ext logical physical expected length flags
>   0       0    34816           30720
>   1 30720 65536            2048 unwritten
>   2   65536    67584            4096 unwritten,eof
> /mnt/mp1/file: 1 extent found
>
> The case is not fragmented. The length of #1 extent is a little bit short,
> but there is no point in doing defragmentation because of the hole
> existence.

Please consider this caseL

Assume for a minute  a large sparse file with lots of holes.  This is
one like a VM might create.  And that in the middle of the file is a
block_group with a hole in front of it and a hole after it.  Assume it
is 1 extent long exactly, and that one extent is a maximum extent (ie
128 MB with 4KB blocks I believe).

Now assume the VM writes a single short extent at the end of
block_group.  Your logic says full_extent+small_extent, no need to
defrag because its already optimal.

I agree with that.

Now assume instead of the block_group being extended at the tail, it
is extended at the head.

Now you have:

   small_extent+full_extent

Your logic as I understand it will score that as not as good as the
first case.  I disagree,  Both are optimally defragged and both should
get the same score.

>
> Regards,
> Kazuya Mio

Greg
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