On Thu, Oct 04, 2007 at 07:27:19AM -0700, cyjoyp wrote:
>
> Hi there,
> I am a beginner in learning XFS file system.. I have a doubt, dont
> know whether is silly..If you could help me with this...Thank you..
>
> An extent is 128 bit in size and uses the following struct,
>
> typedef struct xfs_bmbt_irec {
> xfs_fileoff_t br_startoff;
> xfs_fsblock_t br_startblock;
> xfs_filblks_t br_blockcount;
> xfs_exntst_t br_state;
> }
That's the unpacked, in-memory format. The on disk format for
extents is converted by xfs_bmbt_set_allf/xfs_bmbt_disk_set_allf().
>
> I have got in to the BMAP position after traversing the BTREE..
> The BMAP has a leaf and no other siblings..
> I have the extents for directory entries stored in the BMAP..
> This is where is struck badly..
>
> Now I have a 16 byte value from which I can calculate the block count
> ,AG number ,etc...
> 00 00 00 01 00 00 02 00 00 00 00 00 02 A0 00 02
If that is the start of the block, theres a block header first, right?
i.e.:
/*
* * Bmap root header, on-disk form only.
* */
typedef struct xfs_bmdr_block {
__be16 bb_level; /* 0 is a leaf */
__be16 bb_numrecs; /* current # of data records */
} xfs_bmdr_block_t;
So the above woul dbe telling me that this is a level zero block
(a leaf), with one record. Then there's the first record in packed
format....
> In this case I lan up some where else instead going to the block of
> directory entries..
> This br_startoff (logical offset) is causing a difference...I am going
> wrong somwhere in this case..
unpack the extent record first before decoding it. Perhaps you should
be using xfs_db to look at your disk structures rather than trying
to manually decode it from hex dumps. i.e.:
# xfs_db -r -c "inode 128" -c "p core.size core.nblocks core.format u.bmx" -c
"dblock 0" -c "type dir2" -c p /dev/sdb8
core.size = 4096
core.nblocks = 1
core.format = 2 (extents)
u.bmx[0] = [startoff,startblock,blockcount,extentflag] 0:[0,132,1,0]
bhdr.magic = 0x58443242
bhdr.bestfree[0].offset = 0x618
bhdr.bestfree[0].length = 0x770
bhdr.bestfree[1].offset = 0x370
bhdr.bestfree[1].length = 0x128
bhdr.bestfree[2].offset = 0x88
bhdr.bestfree[2].length = 0x58
bu[0].inumber = 128
bu[0].namelen = 1
bu[0].name = "."
bu[0].tag = 0x10
bu[1].inumber = 128
bu[1].namelen = 2
bu[1].name = ".."
bu[1].tag = 0x20
bu[2].inumber = 131
bu[2].namelen = 3
bu[2].name = "tmp"
bu[2].tag = 0x30
bu[3].inumber = 132
bu[3].namelen = 11
bu[3].name = "syscalltest"
bu[3].tag = 0x40
......
Note that this will print the entire block, including unused space with
it's stale contents.
Cheers,
Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group
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