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Date: Sun, 4 Jun 2017 17:18:29 +0300
From: Slawa Olhovchenkov <slw@zxy.spb.ru>
To: Warner Losh <imp@bsdimp.com>
Cc: Allan Jude <allanjude@freebsd.org>,
 FreeBSD Current <freebsd-current@freebsd.org>
Subject: Re: Time to increase MAXPHYS?
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On Sat, Jun 03, 2017 at 11:49:01PM -0600, Warner Losh wrote:

> > Netflix runs MAXPHYS of 8MB. There's issues with something this big, to be
> > sure, especially on memory limited systems. Lots of hardware can't do this
> > big an I/O, and some drivers can't cope, even if the underlying hardware
> > can. Since we don't use such drivers at work, I don't have a list handy
> > (though I think the SG list for NVMe limits it to 1MB). 128k is totally
> > reasonable bump by default, but I think going larger by default should be
> > approached with some caution given the overhead that adds to struct buf.
> > Having it be a run-time tunable would be great.
> >
> 
> Of course 128k is reasonable, it's the current default :). I'd mean to say
> that doubling would have a limited impact. 1MB might be a good default, but
> it might be too big for smaller systems (nothing says it has to be a MI
> constant, though). It would be a perfectly fine default if it were a
> tunable.

Some cloud providers limit IOPs per VM, for this cases MAXPHYS must be
large as posible.