I'll chime in as a large scale operator, and a strong proponent of ceph-volume.
Ceph-disk wasn't accomplishing what was needed with anything other than
vanilla use cases (even then, still kind of broken). I'm not going to re-hash
Sage's valid points too much, but trying to manipulate the old ceph-disk to
work with your own LVM (or other block manager). As far as the pain of
doing something new goes, yes, sometimes moving to newer more flexible
methods results in a large amount of work. Trust me, I feel that pain when
we're talking about things like ceph-volume, bluestore, etc, but these
changes are not made without reason.
As far as LVM performance goes, I think that's well understood in the larger
Linux community. We accept that minimal overhead to accomplish some
of the setups that we're interested in, such as encrypted, lvm-cached
OSDs. The above is not a trivial thing to do using ceph-disk. We know, we
run that in production, at large scale. It's plagued with problems, and since
it's done without Ceph itself, it is difficult to tie the two together. Having
it
managed directly by Ceph, via ceph-volume makes much more sense.
We're not alone in this, so I know it will benefit others as well, at the cost
of technical expertise.
There are maintainers now for ceph-volume, so if there's something you
don't like, I suggest proposing a change.
Warren Wang
On 6/8/18, 11:05 AM, "ceph-users on behalf of Konstantin Shalygin"
<ceph-users-boun...@lists.ceph.com on behalf of k0...@k0ste.ru> wrote:
> - ceph-disk was replaced for two reasons: (1) It's design was
> centered around udev, and it was terrible. We have been plagued for years
> with bugs due to race conditions in the udev-driven activation of OSDs,
> mostly variations of "I rebooted and not all of my OSDs started." It's
> horrible to observe and horrible to debug. (2) It was based on GPT
> partitions, lots of people had block layer tools they wanted to use
> that were LVM-based, and the two didn't mix (no GPT partitions on top of
> LVs).
>
> - We designed ceph-volome to be *modular* because antipicate that there
> are going to be lots of ways that people provision the hardware devices
> that we need to consider. There are already two: legacy ceph-disk devices
> that are still in use and have GPT partitions (handled by 'simple'), and
> lvm. SPDK devices where we manage NVMe devices directly from userspace
> are on the immediate horizon--obviously LVM won't work there since the
> kernel isn't involved at all. We can add any other schemes we like.
>
> - If you don't like LVM (e.g., because you find that there is a measurable
> overhead), let's design a new approach! I wouldn't bother unless you can
> actually measure an impact. But if you can demonstrate a measurable cost,
> let's do it.
>
> - LVM was chosen as the default appraoch for new devices are a few
> reasons:
> - It allows you to attach arbitrary metadata do each device, like which
> cluster uuid it belongs to, which osd uuid it belongs to, which type of
> device it is (primary, db, wal, journal), any secrets needed to fetch it's
> decryption key from a keyserver (the mon by default), and so on.
> - One of the goals was to enable lvm-based block layer modules beneath
> OSDs (dm-cache). All of the other devicemapper-based tools we are
> aware of work with LVM. It was a hammer that hit all nails.
>
> - The 'simple' mode is the current 'out' that avoids using LVM if it's not
> an option for you. We only implemented scan and activate because that was
> all that we saw a current need for. It should be quite easy to add the
> ability to create new OSDs.
>
> I would caution you, though, that simple relies on a file in /etc/ceph
> that has the metadata about the devices. If you lose that file you need
> to have some way to rebuild it or we won't know what to do with your
> devices. That means you should make the devices self-describing in some
> way... not, say, a raw device with dm-crypt layered directly on top, or
> some other option that makes it impossible to tell what it is. As long as
> you can implement 'scan' and get any other info you need (e.g., whatever
> is necessary to fetch decryption keys) then great.
Thanks, I got what I wanted. It was in this form that it was necessary
to submit deprecations to the community: "why do we do this, and what
will it give us." As it was presented: "We kill the tool along with its
functionality, you should use the new one as is, even if you do not know
what it does."
Thanks again, Sage. I think this post should be in ceph blog.
k
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