problem with backup of changing file

Hi Denis and all Dar lovers.




I have faced a serious problem with backup of my /data directory. I am not 
able to overcome the problem.

I am using pipe over nc  -

dar -c - -R /data |nc lto 5000




machine lto reads 


nc -l -p 5000|mbuffer -P 80 -m 1G > betelgeuse-data.1.dar




I am using mbuffer over screen as I can always check the amount of 
transfered data.

I do not think the porblem is here - probably the dar -c itself would break 
as well 





after writting 139GB (shown by mbuffer) and looking at size of file 
betelgeuse-data.1.dar

everything stops with message 





WARNING! File modified while reading it for backup. No more retry for that
file to not exceed the wasted byte limit. File is
/data/docker/devicemapper/devicemapper/data





And the enter will not help to continue ...




The problem is the docker devicemapper/data as it is a kind of a block file 
specially handled by docker. But for OS it should be just another file (the 
tar goes over it smoothly).

It is about 100GB but does not change size - only the date is updated after 
ls -la it is current.

root@betelgeuse:devicemapper# ls -la data 
-rw------- 1 root root 107374182400 Aug 17 17:42 data





Is there some option how I can tell the dar to write what is available and 
continue ?




Now it stops and does not react .. No data are flowing through the nc .




Thanks a lot 


Petr Skoda