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 === PGP key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- In my day, we didn't have fancy high numbers. We had --- "nothing", "one", "twain" and "multitudes".
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