linux-ext4 - Re: Online defragmentation and ext4migrate

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Message-ID: <20070521103828.GB29416@atrey.karlin.mff.cuni.cz>
Date:	Mon, 21 May 2007 12:38:28 +0200
From:	Jan Kara <jack@...e.cz>
To:	Takashi Sato <sho@...s.nec.co.jp>
Cc:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	linux-ext4@...r.kernel.org
Subject: Re: Online defragmentation and ext4migrate

  Hello,

> >While doing online defragmentation do we move the blocks corresponding to 
> >extent index ? The reason why i am asking this is to understand the
> >usefulness of doing a ext4migrate followed by defrag. I understand that 
> >defragmentation in general will improve the performance. But with respect 
> >to ext4migrate we are not touching the data blocks. Instead we build the 
> >extent map and if that requires to have an extent index block then we 
> >allocate one. I am trying to understand what would be the performance 
> >impact of this and whether doing a defrag really improve the performance.
> 
> I think converting a file to extents has the benefit for the performance of
> block searching. If we want to improve also the performance of  reading
> file data, we have to run the defrag after that.
  Yes. On the other hand I believe that some people would like to use
defragmentation but stay with ext3. For them conversion to extents is
no-go.

> >Also looking at the version 0.4 I see that defrag ioctl only work if we 
> >have EXT4_EXTENTS_FL flag set. What are the plans for making defrag work 
> >with indirect block map inode ?
> 
> Unfortunately, my defrag doesn't support an indirect block file.
> But we can reduce fragments in the file with the defrag just after
> ext4migrate.
> 
> In my opinion, to keep the ioctl simple and small is very important
> for ease of maintenance.  So I would rather not support indirect block
> files in the ioctl.  Instead, I can add the call of the migration
> ioctl to my defrag tool in order to defragment indirect block files.
> How do you think of it?
  Yes that could be useful but I don't think it's a complete solution
for people that don't want to migrate.

								Honza
-- 
Jan Kara <jack@...e.cz>
SuSE CR Labs
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