Re: btrfs defrag questions

[Adam Borowski <kilobyte@angband.pl>]

On Sun, Jul 03, 2016 at 04:15:02PM +0200, Henk Slager wrote:
> >> Provided that Dropbox is running in the system, does it mean that it
> >> cannot be defagmented?
> >
> > That is probably true. Files that are mapped into memory (like running
> > executables) cannot be changed on disk. You could make a copy of that
> > file, remove the original, and rename the new into place. As long as
> > the executable is running it will stay on disk but you can now
> > defragment the file and next time dropbox is started it will use the
> > new one.
> 
> I get:
> ERROR: cannot open ./dropbox: Text file busy
> 
> when I run:
> btrfs fi defrag -v ./dropbox
> 
> This is with kernel 4.6.2 and progs 4.6.1, dropbox running and mount
> option compress=lzo

This is the same thing as with dedupe: the kernel requires you to have the
file opened for writing despite there being no direct reasons for this.
Defragging is not a write operation in POSIX sense: it doesn't alter the
file's contents in any way.

I think it'd be good to relax this requirement to check whether the user
_could_ open the file for writing (ie, cap or w permissions).

-- 
An imaginary friend squared is a real enemy.