[Bacula-users] Need help with blocking factor and speeds

All,
I apologize if this is a rehash of previous threads. I've tried searching the archives for an answer, but am not fully understanding what this means for me.

I am running Bacula 2.4.2 on Solaris 10_x86. I am using an Exabyte Magnum 224 tape library with IBM Ultrium LTO-3 tape drive. I currently have Bacula configured for variable block sizes.
I have been unable to determine or set a default blocking factor or compression on my tape unit. I do however believe that this unit accepts the configuration of the application. I don't know if Bacula, other than by using the /dev/rmt/0cbn device purposefully sets compression. I am under the impression however that Bacula is using its default block size of 65k.

I have been having problems with tape backups taking much longer than I believe they should. I have not yet had to restore from tape and have only done so in testing. In production I have a couple of jobs with these properties

Job1 - 334G to tape - Avg 26.20 Mbytes/sec
Job2 - 1.6T to tape - Avg 13.74 Mbytes/sec

Since I've not figured out if Bacula will tell me the write speed to tape for the job, I've just used the start and end times along with the amount of data to calculate the speed. This does not take into account the time required to spool data. I've turned off data spooling for Job2 in order to compare after this week's run and see what the speed might be.

Testing with tar has shown that using the 65k block size is definitely a performance bottleneck. I used real data (not empty or mkfile generated) for my testing to accurately represent what I usually put to tape (Oracle RMAN backup files).

Here are the numbers:

block size 65 Kbytes
Test #1 (backup) tar > rmt/0cbn - 85 Mb/s
Test #2 (restore) tar > rmt/0cbn - 42 Mb/s

This is similar, though still faster to what I see for Bacula jobs. I think the difference is in the data spooling.

Block size 1024 Kbytes
Test #1 tar > rmt/0cbn - 410 Mb/s
Test #2 tar > rmt/0cbn - 409 Mb/s

Block size 2048 Kbytes
Test #1 tar > rmt/0cbn - 410 Mb/s
Test #2 tar > rmt/0cbn - 408 Mb/s

With these other tests and a much larger blocking factor I see greatly improved performance.

I can tell that I need to change the blocking factor for bacula. However, I'm very confused by the documentation's description of doing so and the consequences. I'd like to know how I should setup my blocking factor so that Bacula uses the tape drive most efficiently and also if I change the blocking factor, how will it affect jobs I currently have on tape or labeled volumes?

Thank you,
Shon

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