On Tue, Nov 03, 2015 at 10:48:53AM +0100, hoper@xxxxxxx wrote:
> Hi,
>
> I'm trying to understand how xfs is storing informations,
> but I guess that my documentation "XFS Filesystem Structure
> 2nd Edition Revision 2" is quite outdated :(
>
> I made an XFS filesystem, then made 60 files in /.
>
> # xfs_db -f /root/small
> xfs_db> inode 128
> xfs_db> addr
> current
> byte offset 32768, length 256
> buffer block 64 (fsbno 8), 8 bbs
> inode 128, dir inode 128, type inode
> xfs_db> p
> core.magic = 0x494e
> core.mode = 040755
> core.version = 2
> core.format = 2 (extents)
> [...]
> next_unlinked = null
> u.bmx[0] = [startoff,startblock,blockcount,extentflag] 0:[0,12,1,0]
>
> I made a small script in python which retrieve all theases information from disk,
> except the "12" (location of the X2DB block = 12*4096). From the manuel, and my
> understanding, this information, 12, should be found just next the di_unlinked.
> (page 43 on the manuel). But from an hex editor, from addr 32768, I got this :
>
> 49 4E 41 ED 02 02 00 00 00 00 00 00 00 00 00 00 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 01
> 56 37 5D 71 2D 26 3D 0B 56 37 5D 6C 02 78 F1 EC 56 37 5D 6C 02 78 F1 EC 00 00 00 00 00 00 10 00
> 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 01 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00
> FF FF FF FF 00 00 00 00 00 00 00 00 00 00 00 00 01 80 00 01 00 00 00 00 00 00 00 00 00 00 00 00
>
> First, 49 4E (IN) that's the begining of the inode core (96 bytes). All informations are here,
> and match the output of xfs_db. But after next_unlinked (FF FF FF FF here), and until next IN,
> I can't find any 12 :( Where is it ?
>
The extent information is encoded in a format that's not easily
interpreted from a raw hex dump. See xfs_iformat_extents() to start:
dp = (xfs_bmbt_rec_t *) XFS_DFORK_PTR(dip, whichfork);
xfs_validate_extents(ifp, nex, XFS_EXTFMT_INODE(ip));
for (i = 0; i < nex; i++, dp++) {
xfs_bmbt_rec_host_t *ep = xfs_iext_get_ext(ifp, i);
ep->l0 = get_unaligned_be64(&dp->l0);
ep->l1 = get_unaligned_be64(&dp->l1);
}
So we start after di_next_unlinked and read in two 64-bit, big endian
fields. That data in your dump above is:
l0: 0x0000000000000000
l1: 0x0000000001800001
Then, these 64-bit fields have a special encoding themselves to map to
actual extent data. From xfs_format.h:
/*
* Bmap btree record and extent descriptor.
* l0:63 is an extent flag (value 1 indicates non-normal).
* l0:9-62 are startoff.
* l0:0-8 and l1:21-63 are startblock.
* l1:0-20 are blockcount.
*/
We see that l0 is completely zeroed, so we can infer that this is a
normal extent and startoff is zero. The block count is l1:0-20, which is
00001b or 1. The start block is (((l0 & 0x1FF) << 43) | (l1 >> 21)),
which is:
((0x0 & 0x1FF) << 43) | (0x1800001 >> 21)
0x0 | 0xC
= 0xC == 12
See __xfs_bmbt_get_all() for the code that implements this decoding.
Brian
> Lot's of thanks to anyone that could give me help on this subject.
>
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