add BIO_NORETRY flag, implement support in ata_da, use in ZFS vdev_geom

[Warner Losh]

On Fri, Nov 24, 2017 at 10:17 AM, Andriy Gapon <avg at freebsd.org> wrote:

> On 24/11/2017 16:57, Scott Long wrote:
> >
> >
> >> On Nov 24, 2017, at 6:34 AM, Andriy Gapon <avg at FreeBSD.org> wrote:
> >>
> >> On 24/11/2017 15:08, Warner Losh wrote:
> >>>
> >>>
> >>> On Fri, Nov 24, 2017 at 3:30 AM, Andriy Gapon <avg at freebsd.org
> >>> <mailto:avg at freebsd.org>> wrote:
> >>>
> >>>
> >>>    https://reviews.freebsd.org/D13224 <https://reviews.freebsd.org/
> D13224>
> >>>
> >>>    Anyone interested is welcome to join the review.
> >>>
> >>>
> >>> I think it's a really bad idea. It introduces a 'one-size-fits-all'
> notion of
> >>> QoS that seems misguided. It conflates a shorter timeout with don't
> retry. And
> >>> why is retrying bad? It seems more a notion of 'fail fast' or so other
> concept.
> >>> There's so many other ways you'd want to use it. And it uses the same
> return
> >>> code (EIO) to mean something new. It's generally meant 'The lower
> layers have
> >>> retried this, and it failed, do not submit it again as it will not
> succeed' with
> >>> 'I gave it a half-assed attempt, and that failed, but resubmission
> might work'.
> >>> This breaks a number of assumptions in the BUF/BIO layer as well as
> parts of CAM
> >>> even more than they are broken now.
> >>>
> >>> So let's step back a bit: what problem is it trying to solve?
> >>
> >> A simple example.  I have a mirror, I issue a read to one of its
> members.  Let's
> >> assume there is some trouble with that particular block on that
> particular disk.
> >> The disk may spend a lot of time trying to read it and would still
> fail.  With
> >> the current defaults I would wait 5x that time to finally get the error
> back.
> >> Then I go to another mirror member and get my data from there.
> >
> > There are many RAID stacks that already solve this problem by having a
> policy
> > of always reading all disk members for every transaction, and throwing
> away the
> > sub-transactions that arrive late.  It’s not a policy that is always
> desired, but it
> > serves a useful purpose for low-latency needs.
>
> That's another possible and useful strategy.
>
> >> IMO, this is not optimal.  I'd rather pass BIO_NORETRY to the first
> read, get
> >> the error back sooner and try the other disk sooner.  Only if I know
> that there
> >> are no other copies to try, then I would use the normal read with all
> the retrying.
> >>
> >
> > I agree with Warner that what you are proposing is not correct.  It
> weakens the
> > contract between the disk layer and the upper layers, making it less
> clear who is
> > responsible for retries and less clear what “EIO” means.  That contract
> is already
> > weak due to poor design decisions in VFS-BIO and GEOM, and Warner and I
> > are working on a plan to fix that.
>
> Well...  I do realize now that there is some problem in this area, both
> you and
> Warner mentioned it.  But knowing that it exists is not the same as
> knowing what
> it is :-)
> I understand that it could be rather complex and not easy to describe in a
> short
> email...
>
> But then, this flag is optional, it's off by default and no one is forced
> to
> used it.  If it's used only by ZFS, then it would not be horrible.
>

Except that it isn't the same flag as what Solaris has (its B_FAILFAST does
something different: it isn't about limiting retries but about failing ALL
the queued I/O for a unit, not just trying one retry), and the problems
that it solves are quite rare. And if you return a different errno, then
the EIO contract is still fulfilled.


> Unless it makes things very hard for the infrastructure.
> But I am circling back to not knowing what problem(s) you and Warner are
> planning to fix.
>

The middle layers of the I/O system are a bit fragile in the face of I/O
errors. We're fixing that.

Of course, you still haven't articulated why this approach would be better,
nor show any numbers as to how it makes things better.

Warner