Re: EXT: [ceph-users] ceph-lvm - a tool to deploy OSDs from LVM volumes

[Alfredo Deza <adeza@redhat.com>]

On Mon, Jun 19, 2017 at 5:58 PM, Sage Weil <sweil@redhat.com> wrote:
> On Mon, 19 Jun 2017, Alfredo Deza wrote:
>> On Mon, Jun 19, 2017 at 4:24 PM, John Spray <jspray@redhat.com> wrote:
>> > On Mon, Jun 19, 2017 at 6:53 PM, Alfredo Deza <adeza@redhat.com> wrote:
>> >>>> * faster release cycles
>> >>>> * easier and faster to test
>> >>>
>> >>> I think having one part of Ceph on a different release cycle to the
>> >>> rest of Ceph is an even more dramatic thing than having it in a
>> >>> separate git repository.
>> >>>
>> >>> It seems like there is some dissatisfaction with how the Ceph project
>> >>> as whole is doing things that is driving you to try and do work
>> >>> outside of the repo where the rest of the project lives -- if the
>> >>> release cycles or test infrastructure within Ceph are not adequate for
>> >>> the tool that formats drives for OSDs, what can we do to fix them?
>> >>
>> >> It isn't Ceph the project :)
>> >>
>> >> Not every tool about Ceph has to come from ceph.git, in which case the
>> >> argument could be flipped around: why isn't ceph-installer,
>> >> ceph-ansible, ceph-deploy, radosgw-agent, etc... all coming from
>> >> within ceph.git ?
>> >
>> > ceph-installer, ceph-deploy and ceph-ansible are special cases because
>> > they are installers, that operate before a particular version of Ceph
>> > has been selected for installation, and might operate on two
>> > differently versioned clusters at the same time.
>>
>> This is a perfect use case for ceph-volume, the OSD doesn't (and in
>> most cases this is true) care what is beneath it, as long
>> as it is mounted and has what it needs to function. The rest is
>> *almost like installation*.
>
> This isn't really true.  ceph-volume (or ceph-disk lvm, or whatever we
> call it) is going to have specific knowledge about how to provision the
> OSD.  When we change the bootstrap-osd caps and change hte semantics of
> 'osd new' (take, for example, teh change we just made from 'osd create' to
> 'osd new'), then ceph-mon, the cephx caps, and ceph-disk all have to
> change in unison.  More concretely, with bluestore we have all kinds of
> choices of how we provision the volumes (what sizes, what options for
> rocksdb, whatever), those opinions will be enshrine in ceph-volume, and
> they will change from version to version... likely in unison with
> bluestore itself (as the code changes the best practice and
> recommendations change with it).
>
> In contrast, I can't think of a reason why ceph-volume without change
> independently of ceph-osd.  There is no bootstrap issue like with
> installation.  And no reason why you would want to run different between
> different versions.
>
>
>
>> > radosgw-agent, presumably (I haven't worked on it) is separate because
>> > it sits between two clusters but is logically part of neither, and
>> > those clusters could potentially be different-versioned too.
>> >
>> > ceph-disk, on the other hand, rides alongside ceph-osd, writes a
>> > format that ceph-osd needs to understand, the two go together
>> > everywhere.  You use whatever version of ceph-disk corresponds to the
>> > ceph-osd package you have.  You run whatever ceph-osd corresponds to
>> > the version of ceph-disk you just used.  The two things are not
>> > separate, any more than ceph-objectstore-tool would be.
>>
>> The OSD needs a mounted volume that has pieces that the OSD itself
>> puts in there. It is a bit convoluted because
>> there are other steps, but the tool itself isn't crucial for the OSD
>> to function, it is borderline an orchestrator to get the volume
>> where the OSD runs ready.
>>
>> >
>> > It would be more intuitive if we had called ceph-disk
>> > "ceph-osd-format" or similar.  The utility that prepares drives for
>> > use by the OSD naturally belongs in the same package (or at the very
>> > least the same release!) as the OSD code that reads that on-disk
>> > format.
>> >
>> > There is a very clear distinction in my mind between things that
>> > install Ceph (i.e. they operate before the ceph packages are on the
>> > system), and things that prepare the system (a particular Ceph version
>> > is already installed, we're just getting ready to run it).
>> > ceph-objectstore-tool would be another example of something that
>> > operates on the drives, but is intimately coupled to the OSDs and
>> > would not make sense as a separately released thing.
>>
>> And ceph-disk isn't really coupled (maybe a tiny corner of it is). Or
>> maybe you can exemplify how those are tied? I've gone through every
>> single step to get an OSD, and although in some cases it is a bit more
>> complex, it isn't more than a few steps (6 in total from our own docs):
>>
>> http://docs.ceph.com/docs/master/rados/operations/add-or-rm-osds/#adding-an-osd-manual
>>
>> ceph-ansible *does* prepare a system for running Ceph, so does
>> ceph-docker. ceph-disk has had some pitfalls that ceph-ansible has to
>> workaround, and has to implement other things as well to be able to
>> deploy OSDs.
>
> Again, I think the 'osd create' -> 'osd new' is a perfect example of
> coupling.  And I anticipate others with bluestore.  For example, when we
> start supporting SPDK for NVMe (kernel bypass) the interface for setting
> that up will likely evolve and will need to match the behavior in
> ceph-volume.
>
> [...]
>
> Perhaps we can look at this from the other angle, though?  Why *should*
> this particular tool be separate?
>
>
>> >>>> Is your argument only to have parity in Ceph's branching? That was
>> >>>> never a problem with out-of-tree tools like ceph-deploy for example.
>> >>>
>> >>> I guess my argument isn't so much an argument as it is an assertion
>> >>> that if you want to go your own way then you need to have a really
>> >>> strong clear reason.
>> >>
>> >> Many! Like I mentioned: easier testing, faster release cycle, can
>> >> publish in any package index, doesn't need anything in ceph.git to
>> >> operate, etc..
>> >
>> > Testing: being separate is only easier if you're only doing python
>> > unit testing.  If you're testing that ceph-disk/ceph-volume really
>> > does its job, then you absolutely do want to be in the ceph tree, so
>> > that you can fire up an OSD that checks that ceph-disk really did it's
>> > job.
>> >
>> > Faster release cycle: we release pretty often.
>>
>> Uh, it depends on what "fast" means for you. 4 months waiting on a
>> ceph-disk issue that was fixed/merged to have ceph-ansible not have that
>> bug is not really fast.
>
> Can you be tell us more about this incident?  We are regular backport
> changes to the stable branches, and have a pretty regular cadence for
> stable release.

I discussed this with Ken at some point, and going through those notes
found a couple of things worth noting.

* keeping/syncing up with feature bits is going to be easier:
Although like I've mentioned this is not that hard to do, it will end
up being one less thing to worry about

* delays in backporting, or releases, is not something that should be
worked around:
In this case, it is clear I am trying to circumvent that to make other
consumer(s) life easier, like in the case with ceph-ansible. Ken
emphasized that issues like those need to bubble up so that leads are
aware. A separate tool doesn't really improve that process.

Since one of the initial agreements for this tool was to be outside of
Ceph to promote faster development to get to a release, it was
surprising to see
push back in something that I didn't see was up for discussion.

So I am still conflicted on ease of (initial) development. This will
make things a bit slower for me, but the offset is that at some point
it would make sense
for the tool to live along the rest of the Ceph components. I will
start working on porting the initial work over, and try to get some
ad-hoc test harness for pull requests (which
would also fix the issue of having to go through make check)

Having said that, I want to reiterate (seems it may have been lost in
the many back and forth) that it has not been my intention to
criticize the Ceph release cycle, or
any of the workflows for backporting features/fixes.

Thanks all for the feedback
>
>> >  We release often
>> > enough to deal with critical OSD and mon bugs.  The tool that formats
>> > OSDs doesn't need to be released more often than the OSD itself.
>>
>> It does need to be released often when the tool is new!
>
> For development, we are doing builds on a continuous basis, with new
> 'master' or branch packages every few hours in most cases.  And all of our
> deployment tools can deploy those test branches...
>
>
>> > I know how backwards that must sound, when you're looking at the
>> > possibility of having a nice self contained git repo, that contains a
>> > pypi-eligible python module, which has unit tests that run fast in
>> > jenkins on every commit.  I get the appeal!  But for the sake of the
>> > overall simplicity of Ceph, please think again, or if you really want
>> > to convert us to a multi-repo model, then make that case for the
>> > project as a whole rather than doing it individually on a bit-by-bit
>> > basis.
>>
>> We can't make the world of Ceph repos to abide today by a multi-repo
>> model. I would need to counter argue you for a few more months :)
>>
>> The examples you give for ceph-disk, and how ceph-disk is today, is why
>> we want to change things.
>>
>> It is not only faster unit tests, or a "nice self contained git repo"
>> just because we want to release to PyPI, we are facing a situation where
>> we need faster development and increased release cycles that we can't
>> get being in an already big repository.
>
> If I'm reading this right, the core reason is "faster development and
> increased release cycles".  Can you explain what that means at a
> practical level?  We build packages all day every day, and don't generally
> need packages at all for development testing.  And any release that
> uses ceph-volume is weeks away, and will be followed up by a regular
> cadence of point releases.  Where is the limitation?
>
> Thanks!
> sage
>
>
>>
>>
>> >
>> > John
>> >
>> >> Even in some cases like pybind, it has been requested numerous times
>> >> to get them on separate package indexes like PyPI, but that has always
>> >> been
>> >> *tremendously* difficult: http://tracker.ceph.com/issues/5900
>> >>>>>  - I agree with others that a single entrypoint (i.e. executable) will
>> >>>>> be more manageable than having conspicuously separate tools, but we
>> >>>>> shouldn't worry too much about making things "plugins" as such -- they
>> >>>>> can just be distinct code inside one tool, sharing as much or as
>> >>>>> little as they need.
>> >>>>>
>> >>>>> What if we delivered this set of LVM functionality as "ceph-disk lvm
>> >>>>> ..." commands to minimise the impression that the tooling is changing,
>> >>>>> even if internally it's all new/distinct code?
>> >>>>
>> >>>> That sounded appealing initially, but because we are introducing a
>> >>>> very different API, it would look odd to interact
>> >>>> with other subcommands without a normalized interaction. For example,
>> >>>> for 'prepare' this would be:
>> >>>>
>> >>>> ceph-disk prepare [...]
>> >>>>
>> >>>> And for LVM it would possible be
>> >>>>
>> >>>> ceph-disk lvm prepare [...]
>> >>>>
>> >>>> The level at which these similar actions are presented imply that one
>> >>>> may be a preferred (or even default) one, while the other one
>> >>>> isn't.
>> >>>>
>> >>>> At one point we are going to add regular disk worfklows (replacing
>> >>>> ceph-disk functionality) and then it would become even more
>> >>>> confusing to keep it there (or do you think at that point we could split?)
>> >>>>
>> >>>>>
>> >>>>> At the risk of being a bit picky about language, I don't like calling
>> >>>>> this anything with "volume" in the name, because afaik we've never
>> >>>>> ever called OSDs or the drives they occupy "volumes", so we're
>> >>>>> introducing a whole new noun, and a widely used (to mean different
>> >>>>> things) one at that.
>> >>>>>
>> >>>>
>> >>>> We have never called them 'volumes' because there was never anything
>> >>>> to support something other than regular disks, the approach
>> >>>> has always been disks and partitions.
>> >>>>
>> >>>> A "volume" can be a physical volume (e.g. a disk) or a logical one
>> >>>> (lvm, dmcache). It is an all-encompassing name to allow different
>> >>>> device-like to work with.
>> >>>
>> >>> The trouble with "volume" is that it means so many things in so many
>> >>> different storage systems -- I haven't often seen it used to mean
>> >>> "block device" or "drive".  It's more often used to describe a logical
>> >>> entity.  I also think "disk" is fine -- most people get the idea that
>> >>> a disk is a hard drive but it could also be any block device.
>> >>
>> >> If your thinking is that a disk can be any block device then yes, we
>> >> are at opposite ends here of our naming. We are picking a
>> >> "widely used" term because it is not specific. "disk" sounds fairly
>> >> specific, and we don't want that.
>> --
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