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

[Andriy Gapon]

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.
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.


-- 
Andriy Gapon