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Date: Sun, 4 Jun 2017 17:50:46 +0900
From: Tomoaki AOKI <junchoon@dec.sakura.ne.jp>
To: freebsd-current@freebsd.org
Cc: Hans Petter Selasky <hps@selasky.org>
Subject: Re: Time to increase MAXPHYS?
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On Sun, 4 Jun 2017 09:52:36 +0200
Hans Petter Selasky <hps@selasky.org> wrote:

> On 06/04/17 09:39, Tomoaki AOKI wrote:
> > Hi
> > 
> > One possibility would be to make it MD build-time OTIONS,
> > defaulting 1M on regular systems and 128k on smaller systems.
> > 
> > Of course I guess making it a tunable (or sysctl) would be best,
> > though.
> > 
> 
> Hi,
> 
> A tunable sysctl would be fine, but beware that commonly used firmware 
> out there produced in the millions might hang in a non-recoverable way 
> if you exceed their "internal limits". Conditionally lowering this 
> definition is fine, but increasing it needs to be carefully verified.
> 
> For example many USB devices are only tested with OS'es like Windows and 
> MacOS and if these have any kind of limitation on the SCSI transfer 
> sizes, it is very likely many devices out there do not support any 
> larger transfer sizes either.
> 
> --HPS

Hmm, so making it tunable (or sysctl) as Warner noted would allow
drivers to use quirks to workaround, i.e., QUIRKS_MAXPHYS_128K.

> 
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-- 
Tomoaki AOKI    <junchoon@dec.sakura.ne.jp>