You have a Synology NAS (Synology DS415+ running DSM 6.1-15047)

You want to change the volume file system from ext4 to btrfs

You have 4.1T of data

You have to completely remove the data and then break and rebuild the volume to convert it

You hook up an ethernet cable from it to another NAS (Netgear ReadyNAS)

You enable ssh on the ReadyNAS

You run rsync

# on synology logged in as root
cd /volume1
# 10.99.99.1 is the Netgear ReadyNAS
rsync -azP ./* 10.99.99.1:/jmits-vol1/backup1/

The 4.1T gets bigger on the target volume. You discover that the volume contains a heap of rsnapshot / backintime files that employ hardlinks so backup data isn't recopied at every backup.

You re-run rysnc with hopefully some optimizations and a fix to transfer the hardlinked files without creating new ones (and chewing up space) on the target volume

rsync -azP -H --numeric-ids ./* 10.99.99.1:/jmits-vol1/backup1/

You think why is the copy taking days

You look at the transfer speed of the rsync copy

data/home/Downloads/COMPANY1/Downloads/company1svn.gz
   403865600  77%   13.13MB/s    0:00:08 (total: 16%)

You discover it is running at 13~ ish MB/s

You goto the CalcTool at http://www.calctool.org/CALC/prof/computing/transfer_time

You wonder why these products have such terrible throughput.

You go to https://gist.github.com/KartikTalwar/4393116