NBU Multiplexing Throughput Slower than Separate Stream Throughput

[ngreene]

Senario:
SAN file system with 20GB of files (each anywhere from 20-100MB)
Netbackup Policy that splits this into 4 streams /u25/[A-C]*  etc.
When each stream is set to run to an individual drive (9940B tape drive) I get a throughput of about 30,000KB/sec -- 4 streams to 4 drives -> simple multistreaming
When each stream is set to run to a single drive using 4 way multiplexing (all 4 streams to 1 drive at the same time).  I show a throughput of only about 9,000KB/sec  -- 4 streams to 1 drive -> multistreaming with multiplexing
I am curious as to why the throughput is showing so much slower with multiplexing.  I did look at the bptm log and found that during simple multistreaming I have write_data waiting on full buffer, so I would assume I would get better performance with the multiplexing.


Thanks

[bak]

Multiplexing will send multiple data streams to the same tape device.  This will always be slower than sending those streams to separate tape devices.  Where multiplexing makes sense is in a scenario where you have several slow clients that you multiplex together to a single fast tape device.  The scenario that you describe is not a very good fit for multiplexing because you can get the data off of the client fast enough to drive the tape devices. 

[lu]

When you activate multiplexing, the NBU drive buffers are shared between the 4 streams. So you may have to increase the size or the number of data buffers to get improve the performance. Ex: no mpx: 1024kb buffers for 4 drives => 4096kb total with mpx: 1024kb/number of streams => 1024kb total and only 256kb for each stream

[ngreene]

My reasoning for multiplexing in this manner is because we have so many backups running at the same time that there is massive contention for the tape drives during off hours.  I am trying to reduce this contention.  If I run this simply as a large single stream, then I have a lot of write_data waiting on full buffer in my bptm logs.   So it would seem that multistreaming with multiplexing would allow me to push more data to that single drive by allowing multiple reads from SAN instead of single file reads.