Time to increase MAXPHYS?

[Warner Losh]

On Sat, Jun 3, 2017 at 2:59 PM, Colin Percival <cperciva at tarsnap.com> wrote:

> On January 24, 1998, in what was later renumbered to SVN r32724, dyson@
> 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).
>

MAXPHYS is the largest I/O transaction you can push through the system. It
doesn't matter that the I/O is physical or not. The name is a relic from a
time that NFS didn't exist.

Warner