Re: Why does btrfs defrag work worse than making a copy of a file?

[Liu Bo <bo.li.liu@oracle.com>]

On Wed, Jul 16, 2014 at 12:50:30PM +0200, Sebastian Ochmann wrote:
> On 16.07.2014 09:53, Liu Bo wrote:
> >On Tue, Jul 15, 2014 at 11:17:26PM +0200, Sebastian Ochmann wrote:
> >>Hello,
> >>
> >>I have a VirtualBox hard drive image which is quite fragmented even
> >>after very light use; it is 1.6 GB in size and has around 5000
> >>fragments (I'm using "filefrag" to determine the number of
> >>fragments). Doing a "btrfs fi defrag -f image.vdi" reduced the
> >>number of fragments to 3749. Even doing a "btrfs fi defrag -f -t 1
> >>image.vdi" which should make sure every extent is rewritten
> >>(according to the btrfs-progs 3.14.2 manpage) does not yield any
> >>better result and seems to return immediately. Copying the file,
> >>however, yields a copy which has only 5 fragments (simply doing a cp
> >>image.vdi image2.vdi; sync; filefrag image2.vdi).
> >>
> >>What do I have to do to defrag the file to the minimal number of
> >>fragments possible? Am I missing something?
> >
> >So usually btrfs thinks of an extent whose size is bigger than 256K as a big
> >enough extent.
> >
> >Another possible reason is that there is something wrong with btrfs_fiemap which
> >gives filefrag' a wrong output.
> >
> >Would you please show us the 'filefrag -v' output?
> 
> Sure, I have pasted the output of "filefrag -v" here:
> 
> http://pastebin.com/kcZhVhkc
> 
> However, I think the problem is merely in the documentation (manpage
> of btrfs-filesystem). The description of the "-t" option is
> different in two locations and doesn't make sense in general, I
> think. It is first described as follows:
> 
> "Any extent bigger than threshold given by -t option, will be
> considered already defragged. Use 0 to take the kernel default, and
> use 1 to say every single extent must be rewritten."
> 
> So I used "-t 1" because I thought it will defrag as much as
> possible. However when thinking about it, any extent at least 1 byte
> (or 2 bytes?) in size will be ignored this way, am I correct?
> 
> Further below, the "-t" option is described as follows:
> 
> "-t <size>  defragment only files at least <size> bytes big"
> 
> Here, the option suddenly refers to the file size. In any case,
> doing a "btrfs fi defrag -f -t 10G image.vdi" defragged my file to
> the 5 extents I also get by simply copying the file. I think the
> documentation should be updated to reflect what the -t option
> actually does.

Oops, we've deeply made users confused in this old documents, will update it
then.

thanks,
-liubo

> 
> Best regards
> Sebastian
> 
> 
> >thanks,
> >-liubo
> >
> >>
> >>Kernel version 3.15.5, btrfs progs 3.14.2, Arch Linux.
> >>
> >>Best regards,
> >>Sebastian
> >>--
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