mount options ignored / unclear

mount options ignored / unclear

[Swâmi Petaramesh]

Hello,

(Still in my quest for BTRFS performance)

It is extremely unclear which BTRFS mount options are "filesystem wide" and 
will apply to each and every mountpoint in the BTRFS filesystem, and which 
options can be set per subvolume or per mountpoint.

As far as I can tell, there is no reliable documentation about this, is there 
any ?

(Kernel is 3.13, Fedora)


i.e. I have in fstab :

/dev/sda3 / btrfs subvol=FEDORA,noatime,compress=lzo,autodefrag

...So without suprise I get in /proc/self/mountinfo:

36 1 0:32 /FEDORA / rw,noatime shared:1 - btrfs /dev/sda3 
rw,seclabel,compress=lzo,space_cache,autodefrag


...But then I have in fstab:

/dev/sda3 /home/me/.cache  btrfs   
subvol=/NOCOW/me/.cache,nodatacow,compress=no,noatime

So it is with great suprise the I get in /proc/self/mountinfo:

49 47 0:32 /NOCOW/me/.cache /home/me/.cache rw,noatime shared:33 - btrfs 
/dev/sda3 rw,seclabel,compress=lzo,space_cache,autodefrag


...I'm not suprised to see "space_cache" which I would assume to be FS-wide, 
but what the hell does "autodefrag" do in there, or even worse "compress=lzo", 
where "compress=no" has been exlicitely specified in fstab ?

That's a bug, as far as I can tell.

Kind regards.

-- 
Swâmi Petaramesh <swami@petaramesh.org> http://petaramesh.org PGP 9076E32E

Re: mount options ignored / unclear

[Duncan]

Swâmi Petaramesh posted on Fri, 11 Apr 2014 11:21:55 +0200 as excerpted:

> It is extremely unclear which BTRFS mount options are "filesystem wide"
> and will apply to each and every mountpoint in the BTRFS filesystem, and
> which options can be set per subvolume or per mountpoint.
> 
> As far as I can tell, there is no reliable documentation about this, is
> there any ?

In general, btrfs doesn't _yet_ have the runtime infrastructure to handle 
per-subvolume mount options.  The on-device format and general approach 
in the kernel was designed to allow it, and it's on the roadmap, it just 
hasn't been done... yet.

So at this time, assume all options apply to the entire filesystem 
including all subvolumes unless you know and can demonstrate otherwise.

If you will take a look back at my posts related to nocow files, while I 
suggested a separate subvolume for those files in ordered to prevent 
snapshotting them with the rest of the filesystem, I specifically did 
*NOT* suggest using the nodatacow mount-option on that subvolume even tho 
I knew about it, instead suggesting either still using the nocow file 
attribute, or putting those file on an entirely separate partition, which 
could then be other than btrfs if desired, precisely because AFAIK the 
nodatacow option cannot apply to a single subvolume yet, and for the most 
part, if one's going to use it on the entire filesystem, one might as 
well use some other filesystem instead of btrfs, since without cow and 
checksumming and snapshotting, there's little left to recommend btrfs 
above a more stable filesystem such as ext3/4 or xfs.  Once the per-
subvolume mount-options work as is planned, simply mounting that 
dedicated subvolume nodatacow will be what I'll recommend, while keeping 
standard cow for the main filesystem, but AFAIK that doesn't work yet, so 
I can't recommend it yet.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman

Re: mount options ignored / unclear

[Chris Murphy]

Best as I can tell, the file system independent mount options can be done per subvolume; while the file system specific mount options apply file system wide, i.e. the options use to mount the first instance of the file system are the ones used for subsequent mounts. To change them requires use of -o remount in which case all mounted subvolume inherit the new mount options.


Chris Murphy