Netbackup disk storage fragment size related......

Update that may help:

I did a couple of things (or more ) which may worked.
Had a look at the mp 7.1.0.4 technote.It contained the minimum hardware specs recomended.Stay above those specs.
I upgraded media/msdp servers to much higher specs but i saw minimal difference at rehydration speed.
Still i could see around 45Mbyte/sec at one Lto3 (from 27MByte/sec before).Still no good enough.

Yet the garbadge collection was running in a blink of the eye  and that alone was a good sign.

I optimized the tape drive settings (buffers/number etc).In common local file backup i could see optimal speed.Did alot of restores also to be sure .

Checked the speed of my storage. Used the camel tool (it has a newer name now) and did copy to an other storage (storage not volume at the same hardware) and kept time .Then did the math to find the storage speed.Before the upgrade i had lower throughput but now I was above 120Mbyte/sec (well symantec says 150 but now i can see 65Myte/sec throughput with 117Mbyte/sec to be exact, and that at my lower spec hardware).Either  way you have to be higher than this, It won't work with lower speeds.

Some time after i changed the maximum fragment size of the DDpool from 15360 to 10240.I think it was better but yet not satisfied.
It happened to have in premises an IBM storage expert.And he noticed that the mode the fiber LTO drives (and the tape library) was at L-port (some kind of loop) and not at N-port (point to point with the switch).That setting alone was extra 10-15Mbyte to throughput at traditional backups .

I also expiremented with the settings of the contentrouter.cfg ( msdp pool path\etc\puredisk). The settings you can change are noticed in the 7.1.0.4 release notes.Two significant settings which can fix or destroy your throughput speed are the "ReadBufferSize" and "PrefetchThreadNum".Change one at the time and while no rehydration jobs running do some 30-50GB duplicate from Msdp pool to tape.It is good enough to show you if it works.

I do daily a couple of things also.
I do garbadge collection at the pools and compaction start by hand.
I disable the rehydration to drives while backing-up staff.After the backup ends i start them (in windows scheduler i have a cmd script) but i have a tape storage (you know a copy of the single one i have) with one drive just to be sure that i won't have two or more reading from pool the same time.