The above are just basic single activity tests - and do not corresepond to real world - but they can serve as a minimum base line.
To really get a handle on what the storage is capable of, one would have to use something like the IOMeter tool and model a set of worker threads which emulate the IO patterns of MSDP - but that would take some time.
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Also, are you aware that throughput to physical HDD spindles really tails off rather quickly as they go above 85% occupied. It is a well known phenomenom. So, if your storage array is at or over 85% full then performance can tail off quite dramatically.
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Usually a typical buffer size for LTO4 and LTO5 is 256KB - but I'm not sure what LTO3 was generally capable of. Not all drives are the same. It could be true that some LTO3 drives use 64KB buffer, whereas some other may use 128KB. Research vendor documentation and/or perform some test backups and test restores to/from test/scratch media when the systems are quiet - or make them quiet by disabling all SLPs and backup policies (except for your test policy) and then do some testing.