Increasing MAXPHYS

Julian Elischer julian at
Sun Mar 21 15:46:57 UTC 2010

Alexander Motin wrote:
> Julian Elischer wrote:
>> In the Fusion-io driver we find that the limiting factor is not the
>> size of MAXPHYS, but the fact that we can not push more than
>> 170k tps through geom. (in my test machine. I've seen more on some
>> beefier machines), but that is only a limit on small transacrtions,
>> or in the case of large transfers the DMA engine tops out before a
>> bigger MAXPHYS would make any difference.
> Yes, GEOM is quite CPU-hungry on high request rates due to number of
> context switches. But impact probably may be reduced from two sides: by
> reducing overhead per request, or by reducing number of requests. Both
> ways may give benefits.
> If common opinion is not to touch defaults now - OK, agreed. (Note,
> Scott, I have agreed :)) But returning to the original question, does
> somebody knows real situation when increased MAXPHYS still causes
> problems? At least to make it safe.

well I know we havn't tested our bsd driver yet with MAXPHYS > 128KB
at this time..   Must try that some time :-)

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