On 2015-09-22 10:36, Hugo Mills wrote:
> On Tue, Sep 22, 2015 at 04:23:33PM +0200, David Sterba wrote:
>> On Tue, Sep 22, 2015 at 01:41:31PM +0000, Hugo Mills wrote:
>>> On Tue, Sep 22, 2015 at 03:36:43PM +0200, Holger Hoffstätte wrote:
>>>> On 09/22/15 14:59, Jeff Mahoney wrote:
>>>> (snip)
>>>>> So if they way we want to prevent the loss of raid type info is by
>>>>> maintaining the last block group allocated with that raid type, fine,
>>>>> but that's a separate discussion.  Personally, I think keeping 1GB
>>>>
>>>> At this point I'm much more surprised to learn that the RAID type can
>>>> apparently get "lost" in the first place, and is not persisted
>>>> separately. I mean..wat?
>>>
>>>     It's always been like that, unfortunately.
>>>
>>>     The code tries to use the RAID type that's already present to work
>>> out what the next allocation should be. If there aren't any chunks in
>>> the FS, the configuration is lost, because it's not stored anywhere
>>> else. It's one of the things that tripped me up badly when I was
>>> failing to rewrite the chunk allocator last year.
>>
>> Yeah, right now there's no persistent default for the allocator. I'm
>> still hoping that the object properties will magically solve that.
>
>     There's no obvious place that filesystem-wide properties can be
> stored, though. There's a userspace tool to manipulate the few current
> FS-wide properties, but that's all special-cased to use the
> "historical" ioctls for those properties, with no generalisation of a
> property store, or even (IIRC) any external API for them.
>
>     We're nominally using xattrs in the btrfs: namespace on directories
> and files, and presumably on the top directory of a subvolume for
> subvol-wide properties, but it's not clear where the FS-wide values
> should go: in the top directory of subvolid=5 would be confusing,
> because then you couldn't separate the properties for *that subvol*
> from the ones for the whole FS (say, the default replication policy,
> where you might want the top subvol to have different properties from
> everything else).
Possibly do special names for the defaults and store them there?  In 
general, I personally see little value in having some special 'default' 
properties however.

The way I would expect things to work is that a new subvolume inherits 
it's properties from it's parent (if it's a snapshot), or from the next 
higher subvolume it's nested in.  This would obviate the need for some 
special 'default' properties, and would be relatively intuitive behavior 
for a significant majority of people.