Since you have caching turned off now and have emptied the diskpool (?)
fragmentation should not be an issue.

What is the performance when using only a single LTO4 drive, 1/4 of
30mb/s or 30mb/s?

Check the read performance of the disk with dd or something similar.

What speed do other tape operations get, ie reclamation, backup stg,
dbbackup?

Have you checked errpt for scsi/tape/disk errors?

What kind of HBA is it, 1/2/4/8Gbps? NPIV or native?

How about moving data from tape up to disk, test if that will be limited
to 30mb/s as well.

On 24/02, Dury, John C. wrote:
> We have a 3 TB disk storage pool that until recently, has been filling up 
> partially during backups, and then emptying as the data is migrated to tape. 
> The storage pool is now at 84% and climbing. I have 4 LTO4 Ultrium drives 
> that can't seem to migrate the data fast enough at all. Watching nmon, I am 
> only getting around 30MB/s total when all four drives are in use. They are 
> all assigned to ne HBA because of a lack of slots in the AIX server. I 
> previously had cache turned on for the disk storage pool (it is now off), so 
> I am wondering if the terrible performance is because the disk storage pool 
> is extremely fragmented because cache was turned on for so long. Any ideas 
> how to offload the data to tape faster?
> John