On 4/6/17 4:59 am, Colin Percival wrote:
On January 24, 1998, in what was later renumbered to SVN r32724, dyson@
Add better support for larger I/O clusters, including larger physical
I/O.  The support is not mature yet, and some of the underlying implementation
needs help.  However, support does exist for IDE devices now.
and increased MAXPHYS from 64 kB to 128 kB.  Is it time to increase it again,
or do we need to wait at least two decades between changes?

This is hurting performance on some systems; in particular, EC2 "io1" disks
are optimized for 256 kB I/Os, EC2 "st1" (throughput optimized spinning rust)
disks are optimized for 1 MB I/Os, and Amazon's NFS service (EFS) recommends
using a maximum I/O size of 1 MB (and despite NFS not being *physical* I/O it
seems to still be limited by MAXPHYS).

We increase it in freebsd 8 and 10.3 on our systems,  Only good results.

sys/sys/param.h:#define MAXPHYS (1024 * 1024) /* max raw I/O transfer size */

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