ITEM: FW7840L

Setting up max_coalesce values.

Env: R40

Problem: Customer was trying to change his coelesce values and queue 
        depth and was getting device busy.

Action Taken: Customer had not varyied off his VG so I had him do this
        and then he was able to make the changes. I also gave him
        the following info on setting the max_coalesce value.
                The SSA device driver allows a max_coalesce attribute of up
        to 2M bytes for each hdisk device.  The default value
        for this attribute is 0x20000, ie 131072 bytes.

        This field tip draws to your attention the possibility that,
        when physical memory is fragemented on 4k boundaries within an
        AIX system, it may only be possible to coalesce operations up 
        to a maximum length of 1Mbyte - 4Kbyte.

        If this occurs, the i/o operations involved will be rejected.

        For this reason, users should not set the max_coalesce 
        attributes to any value greater than 0xFF000.  (Generally, 
        increasing the max_coalesce value allows different i/o 
        operations that are in fact contiguous, to be actioned as a 
        single operation - this can give performance benefits, 
        particularly if full stride write operations occur as a 
        consequence of this in a RAID 5 environment).

        Users who do not change the default values for max_coalesce will
        not be affected by this problem.

        This restriction will be fixed in a future release of the SSA
        device drivers.

Action Plan: close



Support Line: Setting up max_coalesce values. ITEM: FW7840L

Dated: June 1998 Category: N/A

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