"Drivers have been made to behave when using a bigger than 128KB maximum IO transfer size"

Francois Tigeot ftigeot at wolfpond.org
Sun May 11 09:06:38 PDT 2014

On Sun, May 11, 2014 at 12:19:57PM +0200, Wojciech Puchar wrote:
> How to turn in on in dragonfly? i mean setting maximum disk I/O to 2 or 
> 4MB - same as i do in FreeBSD to get large performance gain on big files.
> in FreeBSD
> options         MAXPHYS=2097152 (or twice that)
> options         DFLTPHYS=262144

You could try changing the value of MAXPHYS in sys/cpu/${ARCH}/include/param.h

MAXPHYS, the maximum allowed I/O size is still capped at 64KB (i386) or 128KB
(x86_64/amd64) by default.

I experimented for a while with the ahci driver; bigger values helped to
increase linear (dd) performance by up to 20% with fast SSDs but it was unclear
at the time if it would have improved regular filesystem operations.

Even though all storage drivers compile fine with bigger MAXPHYS values it's
also possible some will have issues at runtime.

Francois Tigeot

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