Re: Why do we need these mount options?

[waxhead <waxhead@dirtcellar.net>]

David Sterba wrote:
> Hi,
> 
> On Thu, Jan 14, 2021 at 03:12:26AM +0100, waxhead wrote:
>> I was looking through the mount options and being a madman with strong
>> opinions I can't help thinking that a lot of them does not really belong
>> as mount options at all, but should rather be properties set on the
>> subvolume - for example the toplevel subvolume.
> 
> I agree that some of them should not be there but mount options still
> have their own usecase. They can be set from the outside and are
> supposed to affect the whole filesystem mount lifetime.
> Yes, some of them. But not all, the ones I list for example can 
perfectly well be set on the toplevel subvolume.

> However, they've been used as default values for some operations, which
> is something that points more to what you suggest. And as they're not
> persistent and need to be stored in /etc/fstab is also weighing for
> storage inside the fs.
> 
>> And any options set on a child subvolume should override the parrent
>> subvolume the way I see it.
> 
> Yeah, that's one of the ways how to do it and I see it that way as well.
> Property set closer to the object takes precedence, roughly
> 
> mount < subvolume < directory < file
> 
> but last time we had a discussion about that, the other oppinion was
> that mount options beat everything, perhaps because they can be set from
> the outside and forced to ovrride whatever is on the filesystem.
> 
Well I agree with that. Mount options should beat everything and just 
because of that I think that some mount options should be deprecated and 
instead be set per. subvolume.

>> By having a quick look - I don't see why these should be mount options
>> at all.
>>
>> autodefrag / noautodefrag
>> commit
>> compress / compress-force
>> datacow / nodatacow
>> datasum / nodatasum
>> discard / nodiscard
>> inode_cache / noinode_cache
>> space_cache / nospace_cache
>> sdd / ssd_spread / nossd / no_ssdspread
>> user_subvol_rm_allowed
> 
> So there are historical reasons and interface limitations that led to
> current state and multiple ways to do things.
> 
> Per-inode attributes were originally private ioctl of ext2 that other
> filesystems adopted due to feature parity, and as the interface was
> bit-based, no additional values could be set eg. compression, limited
> number of bits, no precedence, inter-flag dependencies.
> 
Ok thanks, I was not aware of that.

>> Stuff like compress and nodatacow can be set with chattr so there is as
>> far as I am aware three methods of setting compression for example.
>>
>> Either by mount options in fstab, by chattr or by btrfs property set
>>
>> I think it would be more consistent to have one interface for adjusting
>> behavior.
> 
> I agree with that and there's a proposal to unify that into the
> properties as interface once for all, accessible through the extended
> attributes. But there are much more ways how to do that wrong so it
> hasn't been implemented so far.
> 
Good to know, and by the way another nugget of entertainment is that 
with btrfs property set the parameters come after the object. Usually 
command->params->target is IMHO the better way to go. It seems a bit 
backwards.

> A suggestion for an inode flag here and there comes from time to time,
> fixing one problem each time. Repeating that would lead to a mess that
> can be demonstrated on the existing mount options, so we've been there
> and need to do it the right way.
> 
>> As I asked before, the future plan to have different storage profiles on
>> subvolumes seem to have been sneakily(?) removed from the wiki
> 
> I don't think the per-subvolume storage options were ever tracked on
> wiki, the closest match is per-subvolume mount options that's still
> there
> 
> https://btrfs.wiki.kernel.org/index.php/Project_ideas#Per-subvolume_mount_options
> 
Well how about this from our friends archive.org ?
http://web.archive.org/web/20200117205248/https://btrfs.wiki.kernel.org/index.php/Main_Page

Here it clearly states that object level mirroring and striping is 
planned. Maybe I misinterpret this , but I understand this as (amongst 
other things) configurable storage profiles per subvolume.

>> - if that is indeed a dropped goal I can see why it makes sense to
>> keep the mount options, if not I think the mount options should go in
>> favor of btrfs property set.