Re: Dar on LTO

Dear Petr, Denis,

I've followed in 2022 your thread about using DAR for backups on tape 
with much interest.
But never got past the experimental stage.

As you I like to pick it up again.
And although I've slightly less demanding jobs (only 2x 8Tb compressed 
and a few million files)
I still need several tapes to make a full backup and that will increase 
in future.

One part of my data consists of a lot of hard-links made by 'dirvish' in 
differential mode.
That makes for a very slow data rate using tar for backup to tape with 
shoe shining of the drive as consequence.
I think DAR is a modern equivalent of tar and I like the catalogue 
option to mentioning one so I really want to use it.

I have the same hardware (Tandberg LTO6) with a BTRFS raid1 and a ZFS 
raid1 on spinning rust.
In my experiments I noticed that going beyond tar -b256 didn't gave any 
improvements.
Using mbuffer I also tried a lot of different buffer sizes but because 
of the data part with a lot of hard-links
it always emptied the buffer and the tape drive stopped. And big buffers 
take longer to refill so all in all longer backup times.
The maximum data rate of this drive (400Mb/s compressed) is awful and 
only reachable with SSD's or better (and probably big files).

For now I'm thinking of piping dar into mbuffer something like (very 
rudimentary) :

/usr/bin/mbuffer <lot of options> -R160M -H -f -o /dev/st0 < 
<(/usr/local/bin/dar -c - -zlzh4 -R${rootpath} -@${catalogue} 
|/usr/local/bin/dar_split split_output )

I can remember that there was a discussion 'End of Tape' was not 
reaching dar_split this way because of mbuffer.
I like to use mbuffer because of it's logs showing the data rate.
There are remarkable obervations to make looking at those logs and would 
regret loosing that possibility.

As you can see I compress the data outside the drive and limit the write 
speed to 160Mb/s. Perhaps I will have to lower this even more.
The idea is that by compressing it beforehand and using a buffer shoe 
shining is reduced to a minimum.

My question is is there a good example how to use mbuffer, dar and 
dar_split in combination with a tape drive ?
Is it possible to limit the size of dar_split chunks ?

Thanks and with kind regards,

Jonathan.