How do I find the physical block number?

[Aastha Mehta <aasthakm@gmail.com>]

Hello,

I have created a 500GB partition on my HDD and formatted it for btrfs.
I created a file on it.
# echo "tmp data in the tmp file.." > /mnt/btrfs/tmp-file
# umount /mnt/btrfs

Next I want to know the blocks allocated for the file and I used
filefrag for it. I get some information as follows -

# mount -o max_inline=0 /dev/sdc2 /mnt/btrfs
# filefrag -v /mnt/btrfs/tmp-file
Filesystem type is: 9123683e
File size of /mnt/btrfs/tmp-file is 27 (1 block, blocksize 4096)
 ext logical physical expected length flags
   0       0 65924123               1 eof
/mnt/btrfs/tmp-file: 1 extent found

Now, I want to read the same data from the disk directly. I tried the
following -

block 65924123 = byte (65924123*4096) = 270025207808

# dd if=/dev/sdc2 of=tmp-file skip=270025207808 bs=1 count=4096
# cat tmp-file
I cannot read the file's contents but some garbage.

I read somewhere that the physical block number shown in filefrag may
actually be a logical block for the file system and it has an
additional translation to physical block number. So next I tried the
following -

# btrfs-map-logical -l 65924123 /dev/sdc2
mirror 1 logical 65924123 physical 74312731 device /dev/sdc2
mirror 2 logical 65924123 physical 1148054555 device /dev/sdc2

I again tried reading the block 74312731 using the dd command as
above, but it is still not the right block.

I want to know what does the physical block number returned by
filefrag mean, why there are two mappings for the above block number
and how I can find the exact physical disk block number the file
system actually writes to.

My sdc has the following partitions:
   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1            2048   419432447   209715200   83  Linux
/dev/sdc2      1468008448  2516584447   524288000   83  Linux (BTRFS)
/dev/sdc3       419432448  1468008447   524288000   83  Linux

Thanks,
Aastha.