[Help-tar] help request on tar archiving to lto5 tape

[Balazs Balint]

Dear Help-Tar list members,

I am asking for your advices about a tape archiving issue we encountered 
when using tar with a Dell PV-124T autoloader (enclosed tape drive is a 
Quantum Ultrium 5).
We usually archive up to 8 Terabytes of data with mixed-size files ( 
from several bytes to 30 GB/file) using the command:

tar --create --multi-volume 
--listed-incremental=/data/\!Archive/26Aug2015-raids.copy2.txt 
--file=/dev/st0 4Archive/

When doing so, writing to tape is always as fast as expected (80-120 
MBytes/s)

However, upon reading back the archive we get two different options

1., some of the tapes are very fast to extract tar from: extraction 
write speeds top around 120 MBytes/s as expected, no problem

2., for other tapes, tar gives the following warning: "tar: Record size 
1 block". In addition, reading from the tar or extracting from the tar 
is amazingly slow (~8MBytes/s) for these tapes

What I could read so far in this subject, the situation has to do with 
the --blocking-factor which is supposed to be 20 by default. 
Unfortunately, I did not explicitly set such a value when creating tars 
in the first place and it seems that the automatically selected value is 
not always OK. From now on every new tar archiving shall be carried out 
with the explicit call of --blocking-factor-200 that seems to work fine.

However, we have a decent amount of tapes that turned out to be only 
slow-readable. Is there a way to speed up data read from those tapes 
where tar was created with an automatically chosen blocking factor of 1?

From the tar manual I tried command

tar --verbose --extract --read-full-records --blocking-factor=300 --file 
/dev/nst0

but yet the readout speed tops at 8MB/s.
when trying
dd if=/dev/nst0 of=./tape_image the throughput is very slow as well.

Software environment and version data:

System: Ubuntu 14.04.1 LTS
mt-st: 1.1-5ubuntu1
tar:     1.27.1-1

Can you please advice how could I read back data fast back from tar 
written in 1-block records to LTO5 tapes?

Balazs Balint