On 16 April 2014 15:27, Aastha Mehta aasth...@gmail.com wrote:
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/sdc12048 419432447 209715200 83 Linux
/dev/sdc2 1468008448 2516584447 524288000 83 Linux (BTRFS)
/dev/sdc3 419432448 1468008447 524288000 83 Linux
Thanks,
Aastha.
I realized my mistake in using the btrfs-map-logical command. It
should have been
# btrfs-map-logical -l 270025207808 /dev/sdc2
Now, everything works fine. Please ignore my post, except it may be
useful for somebody else needing this information in future.
Thanks,
Aastha.
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