Re: Time to increase MAXPHYS?

On Sat, Jun 03, 2017 at 11:55:51PM -0400, Allan Jude wrote:

> On 2017-06-03 22:35, Julian Elischer wrote:
> > On 4/6/17 4:59 am, Colin Percival wrote:
> >> On January 24, 1998, in what was later renumbered to SVN r32724, dyson_at_
> >> wrote:
> >>> 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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> 
> At some point Warner and I discussed how hard it might be to make this a
> boot time tunable, so that big amd64 machines can have a larger value
> without causing problems for smaller machines.
> 
> ZFS supports a block size of 1mb, and doing I/Os in 128kb negates some
> of the benefit.

16MB