On 2015-11-20 08:27, Hugo Mills wrote:
> On Fri, Nov 20, 2015 at 08:21:31AM -0500, Austin S Hemmelgarn wrote:
>> On 2015-11-20 06:39, Dmitry Katsubo wrote:
>>> If I may add:
>>>
>>> Information for "System"
>>>
>>>    System, DUP: total=32.00MiB, used=16.00KiB
>>>
>>> is also quite technical, as for end user system = metadata (one can call
>>> it "filesystem metadata" perhaps). For simplicity the numbers can be
>>> added to "Metadata" thus eliminating that line as well.
>>>
>>> For those power users who really want to see the tiny details like
>>> "System" and "GlobalReserve" I suggest to implement "-v" flag:
>>>
>>> # btrfs fi usage -v
>> Actually, I really like this idea, one of the questions I get asked
>> when I show people BTRFS is the difference between System and
>> Metadata, and it's not always easy to explain to somebody who
>> doesn't have a background in filesystem development.  For some
>> reason, people seem to have trouble with the concept that the system
>> tree is an index of the other trees.
>
>     Actually, it's not that in the system chunks. :)
>
>     System chunks contain the chunk tree, not the tree of tree roots.
> They're special (and small) because they're listed explicitly by devid
> and physical offset at the end of the superblock, and allow the FS to
> read them first so that it can bootstrap the logical:physical mapping
> table before it starts reading all the other metadata like the tree of
> tree roots (which is "normal" metadata).
I guess my understanding was wrong then.  Thanks for the explanation.