Re: counting fragments takes more time than defragmenting

[Marc MERLIN <marc@merlins.org>]

Hello again,

Just curious, is anyone seeing similar things with big VM images or other
DBs?
I forgot to mention that my vdi file is 88GB.

It's surprising that it took longer to count the fragments than to actually
defragment the file.
Or that it took 3 defrag runs to get down to 11K extents from 104K.

Are others seeing similar things?

Marc

On Thu, Jun 04, 2015 at 05:42:45PM +0900, Marc MERLIN wrote:
> Hi Chris,
> 
> After our quick chat, I gave it a shot on 3.19.6, and things are better
> than last time I tried.
> 
> legolas:/var/local/nobck/VirtualBox VMs# lsattr Win7/
> ---------------C Win7/Logs
> ---------------C Win7/Snapshots
> ---------------C Win7/Win7.vdi
> ---------------C Win7/Win7.png
> ---------------C Win7/autotune1.png
> ---------------C Win7/new_autotune2.png
> ---------------C Win7/Win7.vbox-prev
> ---------------C Win7/Win7.vbox
> 
> But I have snapshots of that subvolume, so obviously that gets
> in the way of disabling COW.
> 
> I had a look, and I have 100K fragments. That took 10mn to figure out:
> 
> legolas:/var/local/nobck/VirtualBox VMs/Win7# filefrag Win7.vdi
> Win7.vdi: 104306 extents found
> 
> This first filefrag run took about 10mn to count all the fragments on my
> SSD. That feels a bit slow, but maybe the userland tool is doing things
> in suboptimal ways.
> 
> Defrag actually worked (mostly) and wasn't too slow. It used to take hours
> not to finish, and now it worked in 3mn:
> legolas:/var/local/nobck/VirtualBox VMs/Win7# time btrfs fi defrag Win7.vdi 
> real	3m43.807s
> user	0m0.000s
> sys	0m44.044s
> 
> This is defintely better than before.
> Note that it's not fully defragged, but close enough. Each subsequent
> run, filefrag is faster, and defrag is still faster than filefrag:
> 
> legolas:/var/local/nobck/VirtualBox VMs/Win7# time filefrag Win7.vdi
> Win7.vdi: 11428 extents found
> real	2m42.090s
> user	0m0.000s
> sys	2m37.308s
> 
> legolas:/var/local/nobck/VirtualBox VMs/Win7# time btrfs fi defrag Win7.vdi 
> real	0m7.483s
> user	0m0.000s
> sys	0m2.672s
> 
> legolas:/var/local/nobck/VirtualBox VMs/Win7# time filefrag Win7.vdi
> Win7.vdi: 11132 extents found
> real	0m22.525s
> user	0m0.000s
> sys	0m22.264s
> 
> It's a bit unexpected that I still have 10k fragments after 2 defrag
> runs, but it's better than 100k :)
> 
> Marc
> -- 
> "A mouse is a device used to point at the xterm you want to type in" - A.S.R.
> Microsoft is to operating systems ....
>                                       .... what McDonalds is to gourmet cooking
> Home page: http://marc.merlins.org/                         | PGP 1024R/763BE901
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-- 
"A mouse is a device used to point at the xterm you want to type in" - A.S.R.
Microsoft is to operating systems ....
                                      .... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/