Re: [PATCH 0/3] Provide more fine grained control over multipathing

[Mike Snitzer]

On Fri, Jun 01 2018 at 10:09am -0400,
Martin K. Petersen <martin.petersen@xxxxxxxxxx> wrote:

> 
> Good morning Mike,
> 
> > This notion that only native NVMe multipath can be successful is utter
> > bullshit.  And the mere fact that I've gotten such a reaction from a
> > select few speaks to some serious control issues.
> 
> Please stop making this personal.

It cuts both ways, but I agree.

> > Imagine if XFS developers just one day imposed that it is the _only_
> > filesystem that can be used on persistent memory.
> 
> It's not about project X vs. project Y at all. This is about how we got
> to where we are today. And whether we are making right decisions that
> will benefit our users in the long run.
> 
> 20 years ago there were several device-specific SCSI multipath drivers
> available for Linux. All of them out-of-tree because there was no good
> way to consolidate them. They all worked in very different ways because
> the devices themselves were implemented in very different ways. It was a
> nightmare.
> 
> At the time we were very proud of our block layer, an abstraction none
> of the other operating systems really had. And along came Ingo and
> Miguel and did a PoC MD multipath implementation for devices that didn't
> have special needs. It was small, beautiful, and fit well into our shiny
> block layer abstraction. And therefore everyone working on Linux storage
> at the time was convinced that the block layer multipath model was the
> right way to go. Including, I must emphasize, yours truly.
> 
> There were several reasons why the block + userland model was especially
> compelling:
> 
>  1. There were no device serial numbers, UUIDs, or VPD pages. So short
>  of disk labels, there was no way to automatically establish that block
>  device sda was in fact the same LUN as sdb. MD and DM were existing
>  vehicles for describing block device relationships. Either via on-disk
>  metadata or config files and device mapper tables. And system
>  configurations were simple and static enough then that manually
>  maintaining a config file wasn't much of a burden.
> 
>  2. There was lots of talk in the industry about devices supporting
>  heterogeneous multipathing. As in ATA on one port and SCSI on the
>  other. So we deliberately did not want to put multipathing in SCSI,
>  anticipating that these hybrid devices might show up (this was in the
>  IDE days, obviously, predating libata sitting under SCSI). We made
>  several design compromises wrt. SCSI devices to accommodate future
>  coexistence with ATA. Then iSCSI came along and provided a "cheaper
>  than FC" solution and everybody instantly lost interest in ATA
>  multipath.
> 
>  3. The devices at the time needed all sorts of custom knobs to
>  function. Path checkers, load balancing algorithms, explicit failover,
>  etc. We needed a way to run arbitrary, potentially proprietary,
>  commands from to initiate failover and failback. Absolute no-go for the
>  kernel so userland it was.
> 
> Those are some of the considerations that went into the original MD/DM
> multipath approach. Everything made lots of sense at the time. But
> obviously the industry constantly changes, things that were once
> important no longer matter. Some design decisions were made based on
> incorrect assumptions or lack of experience and we ended up with major
> ad-hoc workarounds to the originally envisioned approach. SCSI device
> handlers are the prime examples of how the original transport-agnostic
> model didn't quite cut it. Anyway. So here we are. Current DM multipath
> is a result of a whole string of design decisions, many of which are
> based on assumptions that were valid at the time but which are no longer
> relevant today.
> 
> ALUA came along in an attempt to standardize all the proprietary device
> interactions, thus obsoleting the userland plugin requirement. It also
> solved the ID/discovery aspect as well as provided a way to express
> fault domains. The main problem with ALUA was that it was too
> permissive, letting storage vendors get away with very suboptimal, yet
> compliant, implementations based on their older, proprietary multipath
> architectures. So we got the knobs standardized, but device behavior was
> still all over the place.
> 
> Now enter NVMe. The industry had a chance to clean things up. No legacy
> architectures to accommodate, no need for explicit failover, twiddling
> mode pages, reading sector 0, etc. The rationale behind ANA is for
> multipathing to work without any of the explicit configuration and
> management hassles which riddle SCSI devices for hysterical raisins.

Nice recap for those who aren't aware of the past (decision tree and
considerations that influenced the design of DM multipath).

> My objection to DM vs. NVMe enablement is that I think that the two
> models are a very poor fit (manually configured individual block device
> mapping vs. automatic grouping/failover above and below subsystem
> level). On top of that, no compelling technical reason has been offered
> for why DM multipath is actually a benefit. Nobody enjoys pasting WWNs
> or IQNs into multipath.conf to get things working. And there is no flag
> day/transition path requirement for devices that (with very few
> exceptions) don't actually exist yet.
> 
> So I really don't understand why we must pound a square peg into a round
> hole. NVMe is a different protocol. It is based on several decades of
> storage vendor experience delivering products. And the protocol tries to
> avoid the most annoying pitfalls and deficiencies from the SCSI past. DM
> multipath made a ton of sense when it was conceived, and it continues to
> serve its purpose well for many classes of devices. That does not
> automatically imply that it is an appropriate model for *all* types of
> devices, now and in the future. ANA is a deliberate industry departure
> from the pre-ALUA SCSI universe that begat DM multipath.
> 
> So let's have a rational, technical discussion about what the use cases
> are that would require deviating from the "hands off" aspect of ANA.
> What is it DM can offer that isn't or can't be handled by the ANA code
> in NVMe? What is it that must go against the grain of what the storage
> vendors are trying to achieve with ANA?

Really it boils down to how do users pivot to making use of native NVMe
multipath?  By "pivot" I mean these users have multipath experience.
They have dealt with all the multipath.conf and dm-multipath quirks.
They know how to diagnose and monitor with these tools.  They have their
own scripts and automation to manage the complexity.  In addition, the
dm-multipath model of consuming other linux block devices implies users
have full visibility into IO performance across the entire dm-multipath
stack.

So the biggest failing for native NVMe multipath at this moment: there
is no higherlevel equivalent API for multipath state and performance
monitoring.  And I'm not faulting anyone on the NVMe side for this.  I
know how software development works.  The fundamentals need to be
development before the luxury of higher level APIs and tools development
can make progress.

That said, I think we _do_ need to have a conversation about the current
capabilities of NVMe (and nvme cli) relative to piercing through the
toplevel native NVMe multipath device to really allow a user to "trust
but verify" all is behaving as it should.

So, how do/will native NVMe users:
1) know that a path is down/up (or even a larger subset of the fabric)?
   - coupling this info with topology graphs is useful
2) know the performance of each disparate path (with no path selectors
   at the moment it is moot, but it will become an issue)

It is tough to know the end from the beginning.  And I think you and
others would agree we're basically still in native NVMe multipath's
beginning (might not feel like it given all the hard work that has been
done with the NVMe TWIG, etc).  So given things are still so "green" I'd
imagine you can easily see why distro vendors like Red Hat and SUSE are
looking at this and saying "welp, native NVMe multipath isn't ready,
what are we going to do?".

And given there is so much vendor and customer expertise with
dm-multipath you can probably also see why a logical solution is to
try to enable NVMe multipath _with_ ANA in terms of dm-multipath... to
help us maintain interfaces customers have come to expect.

So dm-multipath is thought as a stop-gap to allow users to use existing
toolchains and APIs (which native NVMe multipath is completely lacking).

I get why that pains Christoph, yourself and others.  I'm not liking it
either believe me!

Mike