add BIO_NORETRY flag, implement support in ata_da, use in ZFS vdev_geom
avg at FreeBSD.org
Sat Nov 25 16:58:38 UTC 2017
On 25/11/2017 18:25, Warner Losh wrote:
> On Fri, Nov 24, 2017 at 10:17 AM, Andriy Gapon <avg at freebsd.org
> <mailto: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>
> >>> <mailto:avg at freebsd.org <mailto:avg at freebsd.org>>> wrote:
> >>> https://reviews.freebsd.org/D13224
> <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
> >>> 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
> >>> 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
> >>> 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.
> >> 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
> 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.
Yes, it isn't the same.
I think that illumos flag does even more.
> 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.
What are the middle layers?
> Of course, you still haven't articulated why this approach would be better
Better than what?
> show any numbers as to how it makes things better.
By now, I have. See my reply to Scott's email.
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