Date: Sat, 25 Nov 2017 18:58:27 +0200 From: Andriy Gapon <avg@FreeBSD.org> To: Warner Losh <imp@bsdimp.com> Cc: Scott Long <scottl@samsco.org>, FreeBSD FS <freebsd-fs@freebsd.org>, freebsd-geom@freebsd.org Subject: Re: add BIO_NORETRY flag, implement support in ata_da, use in ZFS vdev_geom Message-ID: <27c9395f-5b3c-a062-3aee-de591770af0b@FreeBSD.org> In-Reply-To: <CANCZdfqiO7tmD%2BcehaeM-RuENY_Bt5Qj3sOgA4ZbY67oDrcHbg@mail.gmail.com> References: <391f2cc7-0036-06ec-b6c9-e56681114eeb@FreeBSD.org> <CANCZdfoE5UWMC6v4bbov6zizvcEMCbrSdGeJ019axCUfS_T_6w@mail.gmail.com> <64f37301-a3d8-5ac4-a25f-4f6e4254ffe9@FreeBSD.org> <39E8D9C4-6BF3-4844-85AD-3568A6D16E64@samsco.org> <c9a96004-9998-c96d-efd7-d7e510c3c460@FreeBSD.org> <CANCZdfqiO7tmD%2BcehaeM-RuENY_Bt5Qj3sOgA4ZbY67oDrcHbg@mail.gmail.com>
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On 25/11/2017 18:25, Warner Losh wrote: > > > On Fri, Nov 24, 2017 at 10:17 AM, Andriy Gapon <avg@freebsd.org > <mailto:avg@freebsd.org>> wrote: > > On 24/11/2017 16:57, Scott Long wrote: > > > > > >> On Nov 24, 2017, at 6:34 AM, Andriy Gapon <avg@FreeBSD.org> wrote: > >> > >> On 24/11/2017 15:08, Warner Losh wrote: > >>> > >>> > >>> On Fri, Nov 24, 2017 at 3:30 AM, Andriy Gapon <avg@freebsd.org > <mailto:avg@freebsd.org> > >>> <mailto:avg@freebsd.org <mailto:avg@freebsd.org>>> wrote: > >>> > >>> > >>> https://reviews.freebsd.org/D13224 > <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 > 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. 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? > nor > show any numbers as to how it makes things better. By now, I have. See my reply to Scott's email. -- Andriy Gapon
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