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SubjectRe: [PATCH] block: BFQ default for single queue devices
On Wed 03-10-18 08:53:37, Linus Walleij wrote:
> On Wed, Oct 3, 2018 at 8:29 AM Paolo Valente <> wrote:
> > So, I do understand your need for conservativeness, but, after so much
> > evidence on single-queue devices, and so many years! :), what's the
> > point in keeping Linux worse for virtually everybody, by default?
> I understand if we need to ease things in as well, I don't intend this
> change for the current merge window or anything, since v4.19
> will notably have this patch:
> commit d5038a13eca72fb216c07eb717169092e92284f1
> Author: Johannes Thumshirn <>
> Date: Wed Jul 4 10:53:56 2018 +0200
> scsi: core: switch to scsi-mq by default
> It has been more than one year since we tried to change the default from
> legacy to multi queue in SCSI with commit c279bd9e406 ("scsi: default to
> scsi-mq"). But due to issues with suspend/resume and performance problems
> it had been reverted again with commit cbe7dfa26eee ("Revert "scsi: default
> to scsi-mq"").
> In the meantime there have been a substantial amount of performance
> improvements and suspend/resume got fixed as well, thus we can re-enable
> scsi-mq without a significant performance penalty.
> Signed-off-by: Johannes Thumshirn <>
> Reviewed-by: Hannes Reinecke <>
> Reviewed-by: Ming Lei <>
> Acked-by: John Garry <>
> Signed-off-by: Martin K. Petersen <>
> I guess that patch can be a bit scary by itself. But IIUC it all went
> fine this time!
> But hey, if that works, that means $SUBJECT patch will enable BFQ on all
> libata devices and any SCSI that is single queue as well, not just
> "obscure" stuff like MMC/SD and UBI, and that is
> indeed a massive crowd of legacy devices. But we're talking
> v4.21 here.
> Johannes, you might be interested in $SUBJECT patch.
> It'd be nice to hear what SUSE people have to add, since they
> are pretty proactive in this area.

So we do have a udev rules in our distro which sets the IO scheduler based
on device parameters (rotational at least, with blk-mq we might start
considering number of queues as well, plus we have some exceptions like
virtio, loop, etc.). So the kernel default doesn't concern us too much as a

I personally would consider bfq a safer default for single-queue devices
(loop probably needs exception) but I don't feel too strongly about it.

Jan Kara <>

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