>>I use LTO Gen 2. Its read/write speed is 15-20 MB/s more than network
speed
>>(100Mbps). So migration is faster than backup

Maybe, but 20% of your 18gb pool is not a lot spare, and I do not think
you will get a continuous migration process anyway. It will reach zero and
then go all the way back to potentially more than 80% depending how big
the file in progress is. To find out exactly what is happening you could
schedule regular say 10 minute through the night "q stg" commands so you
can see exactly what is going on with your disk pool.
John

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