bug#22517: dd byte count report does not correlate with df byte count report

[Bob Gustafson]

Description of problem:

I am trying to copy /root, /home from a single disk (running the os) to a 
bigger raid 1 disk pair running in the same chassis.

fedora is the lvm vg_group of the running system, fedora23 is the lvm vg_group 
of the new disks (raid 1, 2TB disks).

address@hidden user1]# lvscan
  ACTIVE            '/dev/fedora/swap' [15.57 GiB] inherit
  ACTIVE            '/dev/fedora/home' [865.45 GiB] inherit
  ACTIVE            '/dev/fedora/root' [50.00 GiB] inherit
  ACTIVE            '/dev/fedora23/swap' [15.60 GiB] inherit
  ACTIVE            '/dev/fedora23/root' [50.00 GiB] inherit
  ACTIVE            '/dev/fedora23/home' [1.75 TiB] inherit


Below is the command I'm using to copy over one partition to the raid pair:

address@hidden user1]# dd if=/dev/fedora/root of=/dev/fedora23/root
104857600+0 records in
104857600+0 records out
53687091200 bytes (54 GB) copied, 5639.3 s, 9.5 MB/s
address@hidden user1]#


address@hidden user1]# df -h
Filesystem               Size  Used Avail Use% Mounted on
devtmpfs                  16G     0   16G   0% /dev
tmpfs                     16G  392K   16G   1% /dev/shm
tmpfs                     16G  1.8M   16G   1% /run
tmpfs                     16G     0   16G   0% /sys/fs/cgroup
/dev/mapper/fedora-root   50G   27G   20G  58% /
tmpfs                     16G   84K   16G   1% /tmp
/dev/loop0               1.9G  3.0M  1.7G   1% /srv/node/swift_loopback
/dev/mapper/fedora-home  852G   57G  752G   7% /home
/dev/sdc1                477M  215M  233M  48% /boot
tmpfs                    3.2G  8.0K  3.2G   1% /run/user/42
tmpfs                    3.2G   24K  3.2G   1% /run/user/1000

Note that dd says 54GB were copied over into a disk partition of size only 
50GB. No errors during the copy. And the results show that only 27G was in the 
fedora partition to start with. (54GB is double 27GB - this may be a clue)

Another example:

address@hidden user1]# dd if=/dev/fedora/home of=/dev/fedora23/home
1814978560+0 records in
1814978560+0 records out
929269022720 bytes (929 GB) copied, 32437.8 s, 28.6 MB/s
address@hidden user1]#

Taking a look at the data size on the target partitions:

address@hidden user1]# mount /dev/fedora23/root /mnt/rootn
address@hidden user1]# mount /dev/fedora23/home /mnt/homen

address@hidden user1]# du -h /mnt/rootn | tail -1
26G         /mnt/rootn

address@hidden user1]# du -h /mnt/homen | tail -1
57G         /mnt/homen

for the rootn partition, the 26G copied size is consistent with the source size.

For the homen partition, the 57G is consistent with the source size.

However, the count given by the dd copies don't match:

 In the case of the root->rootn copy it is 54G instead of 27G
 in the case of the home->homen copy it is 942G instead of 57G


Version-Release number of selected component (if applicable):

dd is (coreutils) 8.24

How reproducible:

I did two tests - as shown

Steps to Reproduce:
1. create raid 1 pair
2. copy a 3rd single disk partition to a raid1 pair lvm partition
3. do df and compare results.

Actual results:

As shown above in problem narative.

Expected results:

I would think that the sizes reported by dd and df would be comparable, but 
they are not.

Additional info:

See also https://bugzilla.redhat.com/show_bug.cgi?id=1303456