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AIX Versions 3.2 and 4 Performance Tuning Guide

Modifying the SCSI Device Driver max_coalesce Parameter

When there are multiple disk-I/O requests in the SCSI device driver's queue, it attempts to coalesce those requests into a smaller number of large requests. The largest request (in terms of data transmitted) that the SCSI device driver will build is limited by the max_coalesce parameter. Normally, max_coalesce has a value of 64KB.

To make maximum use of striped logical volumes and disk arrays, it may be desirable to increase the size of max_coalesce. To do so, it is necessary to have a stanza in the PdAt ODM database that specifies the new max_coalesce value. If you have already added such a stanza, you can obtain the current version with:

# odmget -q \
"uniquetype=disk/scsi/osdisk AND attribute=max_coalesce" \
PdAt > foo

If there is no such stanza already, use an editor to create the file foo with the following content:

        uniquetype = "disk/scsi/osdisk"
        attribute = "max_coalesce"
        deflt = "0x20000"
        values = "0x20000"
        width = ""
        type = "R"
        generic = ""
        rep = "n"
        nls_index = 0

Note that max_coalesce, in bytes, is expressed as a hexadecimal number. The deflt and values field values of 0x20000 will set max_coalesce to 128KB. Then replace the old stanza in PdAt, if any, with foo , using:

# odmdelete -o PdAt \
-q "uniquetype=/disk/scsi/osdisk AND attribute=max_coalesce"
# odmadd < foo

To put the change into effect, you must rebuild the kernel and reboot, with:

# bosboot -a -d hdisk0
# shutdown -rF

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