Re: [Dar-support] Multiple slices on LTO6 tape

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

Dear Denis,




I must say that I am still confused concerning the slices and information 
you have written below and all what I have read in dar doc.




I understand that the slice created by dar_split has special format which 
allows to treat it in a different way than slices done by dar_xform.




But then I still do not know how to restore the content when I have 
individual slices on separate tapes. Suppose I have all data for my 
subdirectory only on slice 3 (the last one). You say I need only the last 
slice.

So I insert tape with slice 3 and want to extract the catalogue for further 
usage .

How I will do dar -C  - something like   


dd if=/dev/nst0 bs=256k | dar -C  mycat -A - seems not to work.




I know that I must use the generic name of the slices (without the .3.dar) 
to work on archive. But if it is read from a pipe after dd - how to do it ?




Futhermore you have written in a doc that a special control sequences are 
interspersed acrros the tape to allow the reconstruction but the -0 
(sequential) mode must be used...  So why I may not do this to get the 
content of a particular tape with a slice ?




I suppose that still the FAQ should explain in a detail the questiion:




"I have splitted the data by xform (e.g. after netcat ) to several slices . 
Each of them was written to a separate tape. How I will extract the backup ?
"




All of your answers is about dar_split but you say it must be used to create
the (special) slices. What should I do if  did not do it just used the dar_
xform of simply dar with -s and -S option of create ?

How to extract catalog from such tapes ? How to use sequential mode.




Sorry for my ignorance, but I really have spent  a lot of time reading 
various docs on dar web and mail list but still have very weak understanding
what to do ...




So far it looks that I cannot do anything with multiple slices on tape 
except of feeding the drive with all tapes in sequential mode .

I am not even verify it the particular slice is correct (as the -l  does not
work on other then first slice) Extracting the catalogue from the last one 
is also problematic 


is the only answer - use the dar_split (so I have to start with backups from
the scratch ?  I have already written ten or more tapes with slices from 
multiple backups (filesystems)










Best regards, Petr Skoda







---------- Původní e-mail ----------
Od: Denis Corbin <dar...@fr...>
Komu: dar...@li...
Datum: 13. 5. 2022 18:38:45
Předmět: Re: [Dar-support] Multiple slices on LTO6 tape 
"
> 
> 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."



how the command should look like (with dd and pipe - I need dd because of 
larger block) ?





 
" 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 ... "



Yes but then I need still to have all slices on the disk (called backup.1.
dar , backup.2.dar etc.... - 10TB or more) 





 
"
 
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. 
"



How will it get the right info if reading from tape with the other than 
first slice ?- the extract will not accept the data from tape - (except the 
first)?

I.e. when using dd if=/dev/nst0 s=256k|dar -0 -x -g something  it worked on 
tape with backup.1.dar 














 
"
example to restore with the help of a catalogue 
dar -x backup -A isolated ... "
it is written many times in docs - but here I need the name of backup (so it
must be on the  disk and in fact all slices )





 
"
 

> 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. "



ok - so dar_split is something different which creates some header and 
catalogues for each slices/tape ?




 
"
 
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). 
"



so can I read from tape with dar_split (using -0 as well )or not) 


 
"
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. 
"



Would you mind to write a concrete examples of using /dev/nst0 or /dev/tape)



instead of name of the backup ?

Combination with pipe on dd ?


 
"
> 
> 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)."



In fact it did not help in reading the single slice. It only tells me (while
having all slices on disk ) at which slices the subdire pointed by -g is 
written 





 
" 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. "



I understand - but it seems also not to have header recognized by dar 
(except of the first)


 
"
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 "



Ok - but it means I need to feed all slices (tape by tape) until finding the
right one.

Even if I know that my subdir is on a slice number n>1


 
" 


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



yes but mbuffer is printing the progress the data rates etc ... And the 
buffer filling is IMHO important for preventing the tape shoe shining .

I do not want o make slower the feeding the tape what probably does the -r )
- so in writting it is usefull the if the  tape drive does not cope with a 
speed of feeding data. But on LTO on SAS controller the speed of writting is
fast and the tape waits for feeding and e.g. compressing) the data so the 
buffer (large several GB) is needed not the let the tape to stop.

In fact  in my setup the hard disk is slower than tape speed (as it is not 
on dedicated SAS) 


But I will try to make some tests to prove this my subjective feeling.