Re: Multiple slices on LTO6 tape

Le 13/05/2022 à 10:40, Petr Skoda a écrit :
> Dear DAR experts,

Hi Petr,

> 
> 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.

in the mode you have been using, if you want to list the backup content,
dar will only need the last slice. This will lead you to read the whole
3rd slice from tape. Better you can use instead an isolated catalogue of
the backup on tape (something you can do on-fly or afterward). An
isolated catalogue is usually small an does not require to be backed up
to tape as you can recreate one from a backup at any time.

example to create a isolated catalogue:
dar -C isolated -A backup -z ...

If you want to extract a single file from backup, dar will ask the last
slice, then the slice where the file/tape to restore resided. But if you
restore with the help of an isolated catalogue, dar will only need the
slice where is located the file to restore.

example to restore with the help of a catalogue
dar -x backup -A isolated ...


>  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.

You could better use dar+dar_split instead, if this suits your need. The
advantage is that it will not need you any extra storage to restore or
list the backup, but will act a bit like tar, reading the tapes from the
first toward the last up to the point the file's metadata and data to
restore are reached.

Note that you cannot use dar_split at reading time if you have not used
it at creation time. when using dar+dar_split, dar creates a single
slice, slice that dar_split (as you can guess from its name) splits over
different tapes. Thus, at reading time, dar expects a single slice and
not the concatenation of two or more slices, which is what dar_splits
does at reading time (concatenating the content of several tapes).

However you can convert a splited backup to a single sliced backup using
dar_xform and then use dar_split:

dar_xform backup - | dar_split split_output /dev/tape

note that if you had an isolated catalogue from 'backup' it stays a
valid isolated catalog for the single sliced backup generated by
dar_xform, that dar_split has written to several tapes.

> 
> 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.

you can get file content per slide using the -Tslice option while
listing an multi-sliced backup (or an isolated catalogue from this
backup). But you lose this ability when using dar with dar_split as from
dar stand point, there is only one slice.

> 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.

there is not really a table of content per slice, a sliced backup is
still a coherent backup with an table of content at the end of the slice
set.

> 
> 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.

So I understand, I have to add some FAQ or the like about LTO use with
dar, right?

> 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)

Dar is not expecting to have all slices available at any time, you can
use -p option to pause after each (n) created slice(s) and do what is
needed, like move the produced slices to tape then remove it, before
having dar continuing its work. At reading time, dar will ask for the
missing slice and pause, you can then obtain it from tape and let dar
continue once the slice is available.

Of course you can automate this by mean of -E option, so dar will not
pause and you can batch the whole backup process restoration process (if
you have a robot to physically change tapes... ;) ...)


> everything is slow and time consuming (just listing from my 7TB 3 slice
> the content without sequential mode takes several  minutes  to appear) 


What is long is the time to copy (tape and disk I/O). dar_split should
obiously gives faster execution time as this intermediate step is removed.

Isolated catalogues can still help for listing content (and is also nice
to have to backup the internal catalog of a backup, in case of media
corruption).


> and learning the correct options is almost imposible using tape and
> pipes by trial .

you can use named pipes and loopback mounts to simulate small tapes and
validate your dar commands and backup scenario:

In a first terminal:

for x in 1 2 3 ; do
  dd if=/dev/zero of=tape$x bs=1024 count=11264
  mkfs.ext4 tape$x
done

mkfifo pipe
mkdir mnt

mount -o loop tape1 mnt
cat < pipe > mnt/data


In a second terminal:

dar -c - ... | dar_split split_output pipe

When the tape1 filesystem is full, dar_split pauses, you can then
change the "tape" from the first terminal:

(the 'cat' command has ended)
umount mnt/data
mount -o loop tape2 mnt
cat < pipe > mnt/data

then hit return in the second terminal to let dar_split continue

and so on.



> 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.

Yes, you are right, this is missing in FAQ, I will add this. In the
meanwhile I hope these explanation will let you go forward

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

with dar_split you do not need mbuffer if you just want to rate limit
the throughput (see its -r option)

> 
> 
> Best regards,
> 
> Petr Skoda
> 
> 

Cheers,
Denis