Re: [5.0-rc5 regression] "scsi: kill off the legacy IO path" causes 5 minute delay during boot on Sun Blade 2500
From: James Bottomley
Date: Mon Feb 11 2019 - 23:16:08 EST
On Tue, 2019-02-12 at 03:37 +0000, Elliott, Robert (Persistent Memory)
> > -----Original Message-----
> > From: linux-kernel-owner@xxxxxxxxxxxxxxx <linux-kernel-
> > owner@xxxxxxxxxxxxxxx> On Behalf Of Jens Axboe
> > Sent: Monday, February 11, 2019 8:50 PM
> > To: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>; Mikael
> > Pettersson <mikpelinux@xxxxxxxxx>
> > Cc: Linux SPARC Kernel Mailing List <sparclinux@xxxxxxxxxxxxxxx>;
> > linux-block@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-
> > scsi
> > <linux-scsi@xxxxxxxxxxxxxxx>
> > 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.