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

[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).