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Check path status of disk at expansion box.
sccli show configuration
channels ; disks ; logical-drives ;enclosure-status ( status of every loop)
SAMFS can generate IO with small size of block, so can be problem for SATA disks.
( Check what typical size of block).
SUN300G disks sound like 300GB FC disks.
Depending on how many files are in the SAMFS file system, sharing the mm and mr devices on the same RAID array can be a pretty horrible idea. In my opinion and experience, it's almost always better to NEVER put more than one LUN on a RAID array. Period. Putting more than one LUN on an array results in IO contention on that array. And large, unnaturally configured (9+1? Why?) RAID arrays will have problems from the start.
What are the block sizes used on the RAID arrays? It wouldn't surprise me to see that the RAID array on the expansion tray has a very large block size. Larger block sizes are, in general, not better. Especially for SAMFS metadata - which IIRC is something like 8k or 16k blocks.
I suspect what is happening is most of the metadata updates are going to the mm device on the new array, contending with the IO operations on the file data.
How much space is left on each mm device? What does "iostat -sndxz 2" show when you're having the IO problems?