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The hidden parameter _asm_maxio governs the size of the largest I/O that ASM will make to the storage subsystem.
The parameter defaults to 1 MB on ASM 10g, 11g and 12c.
Although the parameter may be increased it does not actually allow I/O to exceed to 1 MB as this value appears to be a hard limit. Setting max I/O to a value less than 1 MB will likely increase latency for scanning operations.