Re: Question about btrfs metadata structure

On 07/25/2012 06:41 PM, Guenther Rasch wrote:

Hi!

I'm writing my bsc thesis about btrfs and I'm analyzing the
btrfs metadata structure at the moment.

I'm using fedora 17 with kernel 3.4 and btrfs-prog from your
git-repository. For an overview I use "btrfs-debug-tree" and
here are my questions:

The root tree contains some inode_items, but I don't understand
the meaning of them:
item 4 key (ROOT_TREE_DIR INODE_ITEM 0) itemoff 3101 itemsize 160
inode generation 3 size 0 block group 0 mode 40555 links 1

item 5 key (ROOT_TREE_DIR INODE_REF 6) itemoff 3089 itemsize 12
inode ref index 0 namelen 2 name: ..

The key for an inode_item is objectid=ROOT_TREE_DIR. ROOT-TREE-DIR is
the inode-Nr.?
What does offset=6 regarding INODE_REF mean? --> item 5

I think you can find the answer by doing grep ROOT_TREE_DIR in btrfs-progs source code. ;)

Also placed in the root tree, I've found entries of
(FREE_SPACE UNTYPED ....)
What does UNTYPED mean in this context and the offset is
the logical address of a block group (I guess...)? This
item has a pointer to an inode_item, is that right?

It's free space cache, which refers to free-space-cache.c in upstream's code.

And last question about this entries:
item 2 key (FS_TREE INODE_REF 6) itemoff 3500 itemsize 17
 inode ref index 0 namelen 7 name: default
...
item 6 key (ROOT_TREE_DIR DIR_ITEM 2378154706) itemoff 3052 itemsize 37
 location key (FS_TREE ROOT_ITEM 18446744073709551615) type 2
 namelen 7 datalen 0 name: default

There is an INODE_REF (again, offset 6...?) pointing to
which inode? I cannot find some...
What's about that offset in location key (FS_TREE....)

Thx in advance for your patience and answers!

Regards
Guenther
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