Re: btrfs filesystem keeps allocating new chunks for no apparent reason

[Kai Krakow <hurikhan77@gmail.com>]

Am Mon, 10 Apr 2017 13:13:39 -0400
schrieb "Austin S. Hemmelgarn" <ahferroin7@gmail.com>:

> On 2017-04-10 12:54, Kai Krakow wrote:
> > Am Mon, 10 Apr 2017 18:44:44 +0200
> > schrieb Kai Krakow <hurikhan77@gmail.com>:
> >  
> >> Am Mon, 10 Apr 2017 08:51:38 -0400
> >> schrieb "Austin S. Hemmelgarn" <ahferroin7@gmail.com>:
> >>  
>  [...]  
>  [...]  
> >>  [...]  
>  [...]  
>  [...]  
> >>
> >> Did you put it in /etc/fstab only for the rootfs? If yes, it
> >> probably has no effect. You would need to give it as rootflags on
> >> the kernel cmdline.  
> >
> > I did a "fgrep lazytime /usr/src/linux -ir" and it reveals only ext4
> > and f2fs know the flag. Kernel 4.10.
> >
> > So probably you're seeing a placebo effect. If you put lazytime for
> > rootfs just only into fstab, it won't have an effect because on
> > initial mount this file cannot be opened (for obvious reasons), and
> > on remount, btrfs seems to happily accept lazytime but it has no
> > effect. It won't show up in /proc/mounts. Try using it in rootflags
> > kernel cmdline and you should see that the kernel won't accept the
> > flag lazytime. 
> The command-line also rejects a number of perfectly legitimate
> arguments that BTRFS does understand too though, so that's not much
> of a test.

Which are those? I didn't encounter any...

> I've just finished some quick testing though, and it looks
> like you're right, BTRFS does not support this, which means I now
> need to figure out what the hell was causing the IOPS counters in
> collectd to change in rough correlation  with remounting (especially
> since it appears to happen mostly independent of the options being
> changed).

I think that noatime (which I remember you also used?), lazytime, and
relatime are mutually exclusive: they all handle the inode updates.
Maybe that is the effect you see?

> This is somewhat disappointing though, as supporting this would
> probably help with the write-amplification issues inherent in COW
> filesystems. --

Well, relatime is mostly the same thus not perfectly resembling the
POSIX standard. I think the only software that relies on atime is
mutt...

-- 
Regards,
Kai

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