Comments - Which disk does innodb_io_capacity impact?
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"innodb_io_capacity and innodb_io_capacity_max control the InnoDB background tasks activity, including merging data from the insert buffer and flushing pages."
https://mariadb.com/kb/en/innodb-system-variables/#innodb_io_capacity
The innodb background task flushes dirty blocks from InnoDB buffer pool to disk. These dirty blocks can be blocks from tables or indexes or the double write buffer? Those are located either in ibdata? or in all *.ibd files. So it is relevant where those files are located...
So in your case I think it is the RAID array.
Thanks Oli, That's exactly what I was looking for.