linux-kernel - Re: [RFC PATCH] block: xfs: dm thin: train XFS to give up on retrying IO if thinp is out of space

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Date:	Thu, 23 Jul 2015 22:34:09 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Eric Sandeen <sandeen@...hat.com>,
	Mike Snitzer <snitzer@...hat.com>, axboe@...nel.dk,
	linux-kernel@...r.kernel.org, xfs@....sgi.com, dm-devel@...hat.com,
	linux-fsdevel@...r.kernel.org, hch@....de
Subject: Re: [RFC PATCH] block: xfs: dm thin: train XFS to give up on
 retrying IO if thinp is out of space

On Fri, Jul 24, 2015 at 09:00:54AM +1000, Dave Chinner wrote:
> On Thu, Jul 23, 2015 at 12:43:58PM -0400, Vivek Goyal wrote:
> > On Thu, Jul 23, 2015 at 03:10:43PM +1000, Dave Chinner wrote:
> > 
> > [..]
> > > I don't think knowing the bdev timeout is necessary because the
> > > default is most likely to be "fail fast" in this case. i.e. no
> > > retries, just shut down.  IOWs, if we describe the configs and
> > > actions in neutral terms, then the default configurations easy for
> > > users to understand. i.e:
> > > 
> > > bdev enospc		XFS default
> > > -----------		-----------
> > > Fail slow		Fail fast
> > > Fail fast		Fail slow
> > > Fail never		Fail never, Record in log
> > > EOPNOTSUPP		Fail never
> > > 
> > > With that in mind, I'm thinking I should drop the
> > > "permanent/transient" error classifications, and change it "failure
> > > behaviour" with the options "fast slow [never]" and only the slow
> > > option has retry/timeout configuration options.  I think the "never"
> > > option still needs to "fail at unmount" config variable, but we
> > > enable it by default rather than hanging unmount and requiring a
> > > manual shutdown like we do now....
> > 
> > I am wondering instead of 4 knobs (fast,slow,never,retry-timeout) can
> > we just do with one knob per error type and that is retry-timout.
> 
> "retry-timeout" == "fail slow". i.e. a 5 minute retry timeout is
> configured as:
> 
> # echo slow > fail_method
> # echo 0 > max_retries
> # echo 300 > retry_timeout

Hi Dave,

I am sure I am missing something but I will anyway ask. Why do we need this
knob "fail_method". Isn't it sort of implied in other two knobs based
on their values.

max_retries=0 retry_timeout=0 implies fail_method=fast.

A non-zero value of max_retries or retry_timeout implies fail_method=slow

A very high value (-1) of either max_retries or retry_timeout implies
fail_method="almost never".

> > retry-timeout=0 (Fail fast)
> > retry-timeout=X (Fail slow)
> > retry-timeout=-1 (Never Give up).
> 
> What do we do when we want to add a different failure type
> with different configuration requirements?

Ok, got it. So we are targettting something very generic so that other
cases can be handled too.

> 
> > Also do we really need this timeout per error type.
> 
> I don't follow your logic here.  What do need a timeout for with
> either the "never" or "fast" failure configurations?

Ignore this. I had misunderstood it.

> 
> > Also would be nice if this timeout was configurable using a mount
> > option. Then we can just specify it during mount time and be done
> > with it.
> 
> That way lies madness.  The error configuration iinfrastructure we
> need is not just for ENOSPC errors on metadata buffers.  We need
> configurable error behaviour for multiple different errors in
> multiple different subsystems (e.g. data IO failure vs metadata
> buffer IO failure vs memory allocation failure vs inode corruption
> vs freespace corruption vs ....).
> 
> And we still would need the sysfs interface for querying and
> configuring at runtime, so mount options are just a bad idea.  And
> with sysfs, the potential future route for automatic configuration
> at mount time is via udev events and configuration files, similar to
> block devices.

Agreed that sysfs provides lots of flexibility here. I guess I was
just thinking in terms of solving this particular issue  we are facing.

> 
> > Idea of auto tuning based on what block device is doing sounds reasonable
> > but that should not be a requirement for this patch and can go in even
> > later. It is one of those nice to have features.
> 
> "this patch"? Just the core infrastructure so far:

I was referring to Mike's patch where we add additional method to block
device operations.

> 
> 11 files changed, 290 insertions(+), 60 deletions(-)
> 
> and that will need to be split into 4-5 patches for review. There's
> a bunch of cleanup that preceeds this, and then there's a patch per
> error type we are going to handle in metadata buffer IO completion.
> IOWs, the dm-thinp autotuning is just a simple, small patch at the
> end of a much larger series - it's maybe 10 lines of code in XFS...

Ok. I will wait for the final patches. 

Thanks
Vivek
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