[Dar-support] Multiple slices on LTO6 tape

[Petr S. <sez...@se...>]

Dear DAR experts,




I am getting stuck with usage dar on LTO6 tapes. After reading a lot of 
documentation and also archive of this list (and FAQ of course) 


I am not able to find solution for my setup.




I have a machine called lto with connected LTO6 SAS tape (linux 10). Using 
the examples from your docs using netcat and dar_xform 


I have created from the remote machine multiple slices called datas.1.dar 
datas2.dar datas.3.dar. The first two are 2.5 TB (exactly 2500000000bytes 
large the no 3 is smaller.




I have saved each slice to one tape using  dd if=datas.1.dar of=/dev/nst0 bs
=256k etc...

I still have the 3 slices on my staging disk (taking almost 7TB) . But I am 
not sure how to list the content , check the consistency and finally extract
without needing such large space in the future. I would like simply to 
restore only some files in the future getting instruction which tape to use 
and than extract  Ideally using some pipes combined with dd if=/dev/nst0 bs=
256k |  dar something -x -

However dar requires all slices together - I am not able to use only e.g. 
datas.2.dar to list only its contents.




When starting with dar I expected that in sequential mode with some flags 
(even the -al does not work here) I will be able to scan what is on given 
slice and get something from it.

But now having more tapes I am even not able to check what is on the tape 
(if it is slice 2 I am not able simply to use 


dd if=/dev/nst0 bs=256k |dar -0 -l  -   to see just for orientation part of 
the listing of content. 





IMHO many people fighting with LTO storages will find dar soon by googling 
and then will be wondering exactly as I am how to use it.

All tutorials are about small files but when you get to TBs and having not 
space for storing all slices  (say 10-20TB of full backup) 


everything is slow and time consuming (just listing from my 7TB 3 slice the 
content without sequential mode takes several  minutes  to appear)  and 
learning the correct options is almost imposible using tape and pipes by 
trial .

So I would appreciate a new FAQ entry explaining how to use LTO tapes 
together with dar  for doing full and incremental backup when not having 
place to feed all slice to a disk to be able to work with such large volume.

(of course the final goal is to have tape on lto machine but extract through
pipes (and perhaps mbuffer) to remote machine back.







Best regards,




Petr Skoda