call for comments -> "ceph-disk" making OSD directories on typos and is inconsistent (useability).

[Owen Synge <osynge@suse.com>]

Dear all,

By default "ceph-disk" will do the following:

# ceph-disk -vvvv prepare  --fs-type xfs --cluster ceph -- /dev/sdk
DEBUG:ceph-disk:Preparing osd data dir /dev/sdk

No block device "/dev/sdk" exists so "ceph-disk" decides a block device
is not wanted and makes a directory for an OSD.

I think as policy "ceph-disk" should not assume by default, that a non
existent target is correct, make a directory for the "disk" type OSD to
reside in, and set it up as a "disk" type OSD.

Hence I proposed this patch:

https://github.com/ceph/ceph/pull/2160

As a second best option I would be happier with an explicit "don’t fail
if target is not present and just make a directory at the target". But
then you get on to the question of deeper directory structures being
handled:

The current behavior with deeper directory structures is currently
inconsistent as this output shows:

# ceph-disk prepare --fs-type xfs --cluster ceph -- /mnt/vdu/vdu
Traceback (most recent call last):
  File "/usr/sbin/ceph-disk", line 2605, in <module>
    main()
  File "/usr/sbin/ceph-disk", line 2583, in main
    args.func(args)
  File "/usr/sbin/ceph-disk", line 1311, in main_prepare
    os.mkdir(args.data)
OSError: [Errno 2] No such file or directory: '/mnt/vdu/vdu'

I think as a third best option would be to only make directories the
"--data-dir" parameter is used, but still suffers the deeper directory
structures question.

I am still unsure if I like the idea of creating directories for deeper
directory structures, as again the potential for typos leading to vastly
different directory paths with a single misplaced character, and for
consistency would rather "ceph-disk" just failed if the target is not
available.

Although I propose failing fast and clearly with a clear error message
when a target does not exist, removing the assumption that all non
existent targets are valid and "disk" based OSD's and to try and make an
"appropriate" directory, I do see 2 issues with this change:

(A) This is a change to the current default behavior so effecting
deployment frameworks.
(B) This would effect "ceph-deploy" which under some circumstances uses
this behavior.

I propose the following patch to mitigate side effect (B).

https://github.com/ceph/ceph-deploy/pull/224

I see no way to resolve issue (A) in general if my proposal for change
is selected.

I have discussed this issue with "alfredodeza" on IRC both privately and
later on the "ceph-devel" IRC channel and he is "really divided here"
hence we decided I would bring this up for discussion on this mailing list.

Best regards

Owen
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