Re: Newbie questions on some of btrfs code...

[Hugo Mills <hugo@carfax.org.uk>]

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On Fri, May 18, 2012 at 04:32:09PM +0300, Alex Lyakas wrote:
> Thank you, Hugo, for the detailed explanation. I am now able to find
> the CHUNK_ITEMs and to successfully locate the file data on disk.
> Can you maybe address several follow-up questions I have?
> 
> # When looking for CHUNK_ITEMs, should I check that their
> btrfs_chunk::type==BTRFS_BLOCK_GROUP_DATA (and not SYSTEM/METADATA
> etc)? Or file extent should always be mapped to BTRFS_BLOCK_GROUP_DATA
> chunk?

   File extents will either be mapped to a data chunk, _or_ the file
data will live inline in the metadata area, following the
btrfs_extent_item. This is probably the trickiest piece of the on-disk
data format to figure out, and I fear that I didn't document it well
enough. Basically, it's non-obvious where inline extents are
calculated, because there's all sorts of awkward-looking type casting
to get to the data.

> # It looks like I don't even need to bother with the extent tree at
> this point, because from EXTENT_DATA in fs tree I can navigate
> directly to CHUNK_ITEM in chunk tree, correct?

   Mmm... possibly. Again, I'm not sure how this interacts with inline
extents.

> # For replicating RAID levels, you said there will be multiple
> CHUNK_ITEMs. How do I find them then? Should I know in advance how
> much there should be, and look for them, considering only
> btrfs_chunk::type==BTRFS_BLOCK_GROUP_DATA? (I don't bother for
> replication at this point, though).

   Actually, thinking about it, there's a single CHUNK_ITEM, and the
stripe[] array holds all of the per-disk allocations that correspond
to that block group. So, for RAID-1, you'll have precisely two
elements in the stripe[] array. Sorry for getting it wrong earlier.

> # If I find in the fs tree an EXTENT_DATA of type
> BTRFS_FILE_EXTENT_PREALLOC, how should I treat it? What does it mean?
> (BTRFS_FILE_EXTENT_INLINE are easy to treat).

   I don't know, sorry.

> # One of my files has two EXTENT_DATAs, like this:
> 	item 14 key (270 EXTENT_DATA 0) itemoff 1812 itemsize 53
> 		extent data disk byte 432508928 nr 1474560
> 		extent data offset 0 nr 1470464 ram 1474560
> 		extent compression 0
> 	item 15 key (270 EXTENT_DATA 1470464) itemoff 1759 itemsize 53
> 		extent data disk byte 432082944 nr 126976
> 		extent data offset 0 nr 126976 ram 126976
> 		extent compression 0
> Summing btrfs_file_extent_item::num_bytes gives
> 1470464+126976=1597440. (I know that I should not be summing
> btrfs_file_extent_item::disk_num_bytes, but num_bytes).
> However, it's INODE_ITEM gives size of 1593360, which is less:
> 	item 11 key (270 INODE_ITEM 0) itemoff 1970 itemsize 160
> 		inode generation 26 size 1593360 block group 0 mode 100700 links 1
> 
> Is this a valid situation, or I should always consider size in
> INODE_ITEM as the correct one?

   Again, I don't know off the top of my head. It's been some time
since I dug into these kinds of details, sorry.

   Hugo.

-- 
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
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