On Thu, Jul 02, 2020 at 07:21:42PM +0800, Qu Wenruo wrote:
>
>
> On 2020/7/2 下午7:08, Rebraca Dejan (BSOT/PJ-ES1-Bg) wrote:
> > Hi Qu,
> >
> > I'm using this structure to get the address of file extent:
> >
> > struct fiemap_extent {
> > __u64 fe_logical; /* logical offset in bytes for the start of
> > * the extent */
> > __u64 fe_physical; /* physical offset in bytes for the start
> > * of the extent */
>
> fe_physical in btrfs is btrfs logical address.
>
> > __u64 fe_length; /* length in bytes for the extent */
> > __u64 fe_reserved64[2];
> > __u32 fe_flags; /* FIEMAP_EXTENT_* flags for this extent */
> > __u32 fe_reserved[3];
> > };
> >
> > And using fe_physical field I verified that it really reflects the offset in filesystem image - I can see that file content begins at this offset.
> > The problem is that I run into some specific case where file content doesn't begin at fe_physical, I rather have something else at this offset.
>
> As said, there is no guarantee that btrfs logical address is mapped 1:1
> on disk.
> It's possible, but never guaranteed.
>
> You need to pass that fe_physical number to btrfs-map-logical to find
> the real on-disk offset.
>
> Thanks,
> Qu
FYI, I have a utility that does this mapping for all extents in a file:
https://github.com/osandov/osandov-linux/blob/master/scripts/btrfs_map_physical.c
$ sudo ./btrfs_map_physical archlinux-2020.07.01-x86_64.iso | column -ts $'\t'
FILE OFFSET FILE SIZE EXTENT OFFSET EXTENT TYPE LOGICAL SIZE LOGICAL OFFSET PHYSICAL SIZE DEVID PHYSICAL OFFSET
0 6811648 0 regular 6815744 304680529920 6815744 1 89898610688
6811648 4096 0 regular 4096 304594616320 4096 1 89812697088
6815744 70250496 0 regular 70254592 419517255680 70254592 1 168228114432
77066240 4096 0 regular 4096 419587510272 4096 1 168298369024
77070336 127647744 0 regular 127647744 443648155648 127647744 1 209538883584
204718080 552960 0 regular 557056 304605401088 557056 1 89823481856
205271040 134217728 0 regular 134217728 491017764864 134217728 1 238654881792
339488768 43814912 0 regular 43814912 419587514368 43814912 1 168298373120
383303680 241664 0 regular 245760 304605958144 245760 1 89824038912
383545344 4096 0 regular 4096 304594620416 4096 1 89812701184
383549440 134217728 0 regular 134217728 517290700800 134217728 1 255264141312
517767168 78376960 0 regular 78381056 451287601152 78381056 1 213957103616
596144128 82284544 0 regular 82284544 517424918528 82284544 1 255398359040
