Date: Fri, 24 Nov 2017 07:57:55 -0700 From: Scott Long <scottl@samsco.org> To: Andriy Gapon <avg@FreeBSD.org> Cc: Warner Losh <imp@bsdimp.com>, 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: <39E8D9C4-6BF3-4844-85AD-3568A6D16E64@samsco.org> In-Reply-To: <64f37301-a3d8-5ac4-a25f-4f6e4254ffe9@FreeBSD.org> References: <391f2cc7-0036-06ec-b6c9-e56681114eeb@FreeBSD.org> <CANCZdfoE5UWMC6v4bbov6zizvcEMCbrSdGeJ019axCUfS_T_6w@mail.gmail.com> <64f37301-a3d8-5ac4-a25f-4f6e4254ffe9@FreeBSD.org>
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> On Nov 24, 2017, at 6:34 AM, Andriy Gapon <avg@FreeBSD.org> wrote: >=20 > On 24/11/2017 15:08, Warner Losh wrote: >>=20 >>=20 >> On Fri, Nov 24, 2017 at 3:30 AM, Andriy Gapon <avg@freebsd.org >> <mailto:avg@freebsd.org>> wrote: >>=20 >>=20 >> https://reviews.freebsd.org/D13224 = <https://reviews.freebsd.org/D13224>; >>=20 >> Anyone interested is welcome to join the review. >>=20 >>=20 >> 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. >>=20 >> So let's step back a bit: what problem is it trying to solve? >=20 > 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=E2=80=99s not a policy that is = always desired, but it serves a useful purpose for low-latency needs. > 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. >=20 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. =20 Scott
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