[ceph-users] Cluster Down from reweight-by-utilization

Kevin Hrpcek kevin.hrpcek at ssec.wisc.edu
Mon Nov 6 14:38:33 PST 2017


An update for the list archive and if people have similar issues in the 
future.

My cluster took about 18 hours after resetting noup for all of the OSDs 
to get to the current epoch. In the end there were 5 that took a few 
hours longer than the others. Other small issues came up during the 
process such as ceph logs filling up /var and memory/swap filling 
probably caused this all to take longer than it should have. Simply 
restarting the OSDs when memory/swap was filling up allowed them to 
catch up faster. The daemons probably generated a bit under 1tb of logs 
throughout the whole process, so /var got expanded.

Once the OSDs all had current epoch I unset noup and let the cluster 
peer/activate PGs. This took another ~6 hours and was likely slowed by 
some of oldest undersized OSD servers not having enough cpu/memory to 
handle it. Throughout the peering/activating I periodically briefly 
unset nodown as a way to see if there were OSDs that were having 
problems and then addressed those.

In the end everything came back and the cluster is healthy and there are 
no existing PG problems. How the reweight triggered a problem this 
severe is still unknown.

A couple takeaways:
- CPU and memory may not be highly utilized in daily operations but is 
very important for large recovery operations. Having a bit more memory 
and cores would have probably saved hours of time from the recovery 
process and may have prevented my problem altogether.
- Slowing the map changes by quickly setting nodown,noout,noup when 
everything is already down will help as well.

Sage, thanks again for your input and advice.

Kevin



On 11/04/2017 11:54 PM, Sage Weil wrote:
> On Sat, 4 Nov 2017, Kevin Hrpcek wrote:
>> Hey Sage,
>>
>> Thanks for getting back to me this late on a weekend.
>>
>> Do you now why the OSDs were going down?  Are there any crash dumps in the
>> osd logs, or is the OOM killer getting them?
>>
>> That's a part I can't nail down yet. OSDs didn't crash, after the reweight-by-utilization OSDs on some of our earlier gen
>> servers started spinning 100% cpu and were overwhelmed. Admittedly these early gen osd servers are undersized on cpu which is
>> probably why they got overwhelmed, but it hasn't escalated like this before. Heartbeats among the cluster's OSDs started
>> failing on those OSDs first and then the osd 100% cpu  problem seemed to snowball to all hosts. I'm still trying to figure out
>> why the relatively small reweighting caused this problem.
>>
>> The usual strategy here is to set 'noup' and get all of the OSDs to catch
>> up on osdmaps (you can check progress via the above status command).  Once
>> they are all caught up, unset noup and let them all peer at once.
>>
>> I tried having noup set for a few hours earlier to see if stopping the moving osdmap target would help but I eventually unset
>> it while doing more troubleshooting. I'll set it again and let it go overnight. Patience is probably needed with a cluster this
>> size. I saw this similar situation and was trying your previous solution
>> http://lists.ceph.com/pipermail/ceph-users-ceph.com/2014-May/040030.html
>>
>>
>> The problem that has come up here in the past is when the cluster has been
>> unhealthy for a long time and the past intervals use too much memory.  I
>> don't see anything in your description about memory usage, though.  If
>> that does rear its head there's a patch we can apply to kraken to work
>> around it (this is fixed in luminous).
>>
>> Memory usage doesn't seem too bad, a little tight on some of those early gen servers, but I haven't seen OOM killing things off
>> yet. I think I saw mention of that patch and luminous handling this type of situation better while googling the issue...larger
>> osdmap increments or something similar if i recall correctly. My cluster is a few weeks away from a luminous upgrade.
> That's good.  You mgiht also try setting nobackfill and norecover just to
> keep the load off the cluster while it's peering.
>
> s

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