Date: Sat, 25 Nov 2017 10:38:21 -0700 From: Warner Losh <imp@bsdimp.com> To: Andriy Gapon <avg@freebsd.org> 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: <CANCZdfo%2BAb3apD6uw5A42V0AsFWAtPCHJg%2BQK2p2c%2Bi8Frhxsw@mail.gmail.com> In-Reply-To: <27c9395f-5b3c-a062-3aee-de591770af0b@FreeBSD.org> 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> <27c9395f-5b3c-a062-3aee-de591770af0b@FreeBSD.org>
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On Sat, Nov 25, 2017 at 9:58 AM, Andriy Gapon <avg@freebsd.org> wrote: > 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 s= o > other > > concept. > > >>> There's so many other ways you'd want to use it. And it uses th= e > 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 wel= l > 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 th= e > 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=E2=80=99s not a policy tha= t 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 wit= h > 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 =E2=80=9CEIO=E2=80=9D= 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 tha= t > it > > solves are quite rare. And if you return a different errno, then the EI= O > > contract is still fulfilled. > > Yes, it isn't the same. > I think that illumos flag does even more. Since it isn't the same, and there's not other systems that do a similar thing, that ups the burden of proof that this is a good idea. > Unless it makes things very hard for the infrastructure. > > But I am circling back to not knowing what problem(s) you and Warne= r > 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? The buffer cache and lower layers of the UFS code is where the problems chiefly lie. > Of course, you still haven't articulated why this approach would be bette= r > > Better than what? Well, anything? > > nor > > show any numbers as to how it makes things better. > > By now, I have. See my reply to Scott's email. I just checked my email, I've seen no such reply. I checked it before I replied. Maybe it's just delayed. Warner
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