Re: [5.0-rc5 regression] "scsi: kill off the legacy IO path" causes 5 minute delay during boot on Sun Blade 2500

James Bottomley
2019-02-12T04:16:05Z

On Tue, 2019-02-12 at 03:37 +0000, Elliott, Robert (Persistent Memory)
wrote:
> > -----Original Message-----
> > From: linux-kernel-owner@vger.kernel.org <linux-kernel-
> > owner@vger.kernel.org> On Behalf Of Jens Axboe
> > Sent: Monday, February 11, 2019 8:50 PM
> > To: James Bottomley <James.Bottomley@HansenPartnership.com>; Mikael
> > Pettersson <mikpelinux@gmail.com>
> > Cc: Linux SPARC Kernel Mailing List <sparclinux@vger.kernel.org>;
> > linux-block@vger.kernel.org; linux-kernel@vger.kernel.org; linux-
> > scsi
> > <linux-scsi@vger.kernel.org>
> > Subject: Re: [5.0-rc5 regression] "scsi: kill off the legacy IO
> > path"
> > causes 5 minute delay during boot on Sun Blade 2500
>
> ...
>
> > > We can either fix this by setting the QUEUE_FLAG_ADD_RANDOM if
> > > there's no 0xB1 page or by setting the default as Jens proposed.
> >
> > I'd recommend just doing my patch, since that'll be the same
> > behavior that SCSI had before.
>
> When blk-mq and scsi-mq were introduced to boost SAS SSDs over
> 1 million IOPS, this was purposely kept off, because it generated
> a noticeable 3-12 us latency blip whenever the "fast_mix" code ran
> out of bits. If Jens' patch changes the default to be enabled for
> all scsi-mq users, that'll be a change for the newer legacy
> scsi-mq users (except for users that don't trust the default and
> always set the value in sysfs).

All high IOPs devices have a device characteristics page, so even if we
default this to ON, it will get turned off for these devices which
should be the desired result regardless of the default setting.

James