I start to think that the root cause of the remapping is just the fact
that the crush rule(s) contain(s) the "step take default" line:
step take default class hdd
My interpretation is that crush simply tries to honor the rule:
consider everything underneath the "default" root, so PGs get remapped
if new hosts are added there (but not in their designated subtree
buckets). The effect (unknown PGs) is bad, but there are a couple of
options to avoid that:
- Use different root names and/or more specific crush rules.
- Use host spec file(s) to place new hosts directly where they belong.
- Set osd_crush_initial_weight = 0 to avoid remapping until everything
is where it's supposed to be, then reweight the OSDs.
Zitat von Eugen Block <eblock@xxxxxx>:
Hi Frank,
thanks for looking up those trackers. I haven't looked into them
yet, I'll read your response in detail later, but I wanted to add
some new observation:
I added another root bucket (custom) to the osd tree:
# ceph osd tree
ID CLASS WEIGHT TYPE NAME STATUS REWEIGHT PRI-AFF
-12 0 root custom
-1 0.27698 root default
-8 0.09399 room room1
-3 0.04700 host host1
7 hdd 0.02299 osd.7 up 1.00000 1.00000
10 hdd 0.02299 osd.10 up 1.00000 1.00000
...
Then I tried this approach to add a new host directly to the
non-default root:
# cat host5.yaml
service_type: host
hostname: host5
addr: 192.168.168.54
location:
root: custom
labels:
- osd
# ceph orch apply -i host5.yaml
# ceph osd tree
ID CLASS WEIGHT TYPE NAME STATUS REWEIGHT PRI-AFF
-12 0.04678 root custom
-23 0.04678 host host5
1 hdd 0.02339 osd.1 up 1.00000 1.00000
13 hdd 0.02339 osd.13 up 1.00000 1.00000
-1 0.27698 root default
-8 0.09399 room room1
-3 0.04700 host host1
7 hdd 0.02299 osd.7 up 1.00000 1.00000
10 hdd 0.02299 osd.10 up 1.00000 1.00000
...
host5 is placed directly underneath the new custom root correctly,
but not a single PG is marked "remapped"! So this is actually what I
(or we) expected. I'm not sure yet what to make of it, but I'm
leaning towards using this approach in the future and add hosts
underneath a different root first, and then move it to its
designated location.
Just to validate again, I added host6 without a location spec, so
it's placed underneath the default root again:
# ceph osd tree
ID CLASS WEIGHT TYPE NAME STATUS REWEIGHT PRI-AFF
-12 0.04678 root custom
-23 0.04678 host host5
1 hdd 0.02339 osd.1 up 1.00000 1.00000
13 hdd 0.02339 osd.13 up 1.00000 1.00000
-1 0.32376 root default
-25 0.04678 host host6
14 hdd 0.02339 osd.14 up 1.00000 1.00000
15 hdd 0.02339 osd.15 up 1.00000 1.00000
-8 0.09399 room room1
-3 0.04700 host host1
...
And this leads to remapped PGs again. I assume this must be related
to the default root. I'm gonna investigate further.
Thanks!
Eugen
Zitat von Frank Schilder <frans@xxxxxx>:
Hi Eugen,
just to add another strangeness observation from long ago:
https://www.spinics.net/lists/ceph-users/msg74655.html. I didn't
see any reweights in your trees, so its something else. However,
there seem to be multiple issues with EC pools and peering.
I also want to clarify:
If this is the case, it is possible that this is partly
intentional and partly buggy.
"Partly intentional" here means the code behaviour changes when you
add OSDs to the root outside the rooms and this change is not
considered a bug. It is clearly *not* expected as it means you
cannot do maintenance on a pool living on a tree A without
affecting pools on the same device class living on an unmodified
subtree of A.
From a ceph user's point of view everything you observe looks
buggy. I would really like to see a good explanation why the
mappings in the subtree *should* change when adding OSDs above that
subtree as in your case when the expectation for good reasons is
that they don't. This would help devising clean procedures for
adding hosts when you (and I) want to add OSDs first without any
peering and then move OSDs into place to have it happen separate
from adding and not a total mess with everything in parallel.
Best regards,
=================
Frank Schilder
AIT Risø Campus
Bygning 109, rum S14
________________________________________
From: Frank Schilder <frans@xxxxxx>
Sent: Thursday, May 23, 2024 6:32 PM
To: Eugen Block
Cc: ceph-users@xxxxxxx
Subject: Re: unknown PGs after adding hosts in
different subtree
Hi Eugen,
I'm at home now. Could you please check all the remapped PGs that
they have no shards on the new OSDs, i.e. its just shuffling around
mappings within the same set of OSDs under rooms?
If this is the case, it is possible that this is partly intentional
and partly buggy. The remapping is then probably intentional and
the method I use with a disjoint tree for new hosts prevents such
remappings initially (the crush code sees the new OSDs in the root,
doesn't use them but their presence does change choice orders
resulting in remapped PGs). However, the unknown PGs should clearly
not occur.
I'm afraid that the peering code has quite a few bugs, I reported
something at least similarly weird a long time ago:
https://tracker.ceph.com/issues/56995 and
https://tracker.ceph.com/issues/46847. Might even be related. It
looks like peering can loose track of PG members in certain
situations (specifically after adding OSDs until rebalancing
completed). In my cases, I get degraded objects even though
everything is obviously still around. Flipping between the
crush-maps before/after the change re-discovers everything again.
Issue 46847 is long-standing and still unresolved. In case you need
to file a tracker, please consider to refer to the two above as
well as "might be related" if you deem that they might be related.
Best regards,
=================
Frank Schilder
AIT Risø Campus
Bygning 109, rum S14
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