Re: btrfs defrag questions

[Dmitry Katsubo <dma_k@mail.ru>]

On 2016-07-01 22:46, Henk Slager wrote:
> (email ends up in gmail spamfolder)
> On Fri, Jul 1, 2016 at 10:14 PM, Dmitry Katsubo <dma_k@mail.ru> wrote:
>> Hello everyone,
>>
>> Question #1:
>>
>> While doing defrag I got the following message:
>>
>> # btrfs fi defrag -r /home
>> ERROR: defrag failed on /home/user/.dropbox-dist/dropbox: Success
>> total 1 failures
>>
>> I feel that something went wrong, but the message is a bit misleading.
>>
>> Provided that Dropbox is running in the system, does it mean that it
>> cannot be defagmented?
> 
> I think it is a matter of newlines in btrfs-progs and/or stdout/stderr mixup.
> 
> You should run the command with -v and probably also with -f, so that
> it gets hopefully clearer what is wrong.

Running with "-v -f" (or just "-v") result the same output:

...
/home/user/.dropbox-dist/dropbox-lnx.x86-5.4.24/select.so
/home/user/.dropbox-dist/dropbox-lnx.x86-5.4.24/grp.so
/home/user/.dropbox-dist/dropbox-lnx.x86-5.4.24/posixffi.libc._posixffi_libcERROR: defrag failed on /home/user/.dropbox-dist/dropbox-lnx.x86-5.4.24/dropbox: Success
.so
/home/user/.dropbox-dist/dropbox-lnx.x86-5.4.24/_functools.so
/home/user/.dropbox-dist/dropbox-lnx.x86-5.4.24/dropbox
/home/user/.dropbox-dist/dropbox-lnx.x86-5.4.24/_csv.so
...

This is not a matter of newlines:

$ grep -rnH 'defrag failed' btrfs-progs
btrfs-progs/cmds-filesystem.c:1021:       error("defrag failed on %s: %s", fpath, strerror(e));
btrfs-progs/cmds-filesystem.c:1161:                       error("defrag failed on %s: %s", argv[i], strerror(e));

> That it fails on dropbox is an error I think, but maybe known: Could
> be mount option is compress and that that causes trouble for defrag
> although that should not happen.

True, compression is enabled.

> You can defrag just 1 file, so maybe you could try to make a reproducible case.

When I run it on one file, it works as expected:

# btrfs fi defrag -r -v /home/user/.dropbox-dist/dropbox-lnx.x86-5.4.24/dropbox
ERROR: cannot open /home/user/.dropbox-dist/dropbox-lnx.x86-5.4.24/dropbox: Text file busy

> What kernel?
> What btrfs-progs?

kernel v4.4.6
btrfs-tools v4.5.2

>> Question #2:
>>
>> Suppose that in above example /home/ftp is mounted as another btrfs
>> array (not subvolume). Will 'btrfs fi defrag -r /home' defragment it
>> (recursively) as well?
> 
> I dont know, I dont think so, but you can simply try.
 
Many thanks, now I see how can I check this. Unfortunately it does not
descend into submounted directories.

-- 
With best regards,
Dmitry