Thanks Matthias.
On Wed, Mar 22, 2023 at 1:51 PM Matthias Muench <mmuench(a)redhat.com> wrote:
Hi Prashant, et. al.,
separating the logs from the DB might be a good thing.
I would second what Frank suggested: local storage. Local to the mon
instances hosts, perhaps just saying that flash is required which shouldn't
be an issue nowadays. This would also give the best latency to avoid
starvation on IOPS in case of the disaster.
Yes, we can achieve this but maybe instead of mon handling these logs we
can delegate this task to mgr daemon.
With redundancy in the instances, data is available,
at least from one of
the mon instance hosts. Relying on pools would assume that communication is
intact even between the actors of the pool. An exclusive pool for just this
only would still depend on the network connection and introducing
additional latency, too.
Rightly said.
The other alternatives sound promising as well, however, I would like to
raise some concerns.
Pushing the logs only to a central location would impose a dependency on
this location in case of a disaster. A disaster could be also in
conjunction with a network issue affecting the connection to outside world.
So, might be an add-on but for troubleshooting rather some kind of
additional challenge.
Eventually consistent distribution of data might be
hard for
troubleshooting. The basic assumption would be that the logs aren't that
important to be available in full in some of the places, as in the
different mon instance hosts. Eventual consistency also would add another
level of trouble to troubleshoot in conjunction with a disaster. Those
interconnection requirements may be void or at least the service may be at
limited availability that might not help to get the data into the place
just in need.
Yes, it will be *SPOF* for log availability if we log to a central
location. We will consider these inputs. Thanks for your inputs.
Kind regards,
-matt
On 22.03.23 14:10, Ernesto Puerta wrote:
Hi Prashant,
Is this move just limited to the impact of the cluster log in the mon
store db or is it part of a larger mon db clean-up effort?
I'm asking this because, besides de cluster log, the mon store db is
currently used (and perhaps abused) also by some mgr modules via:
- set_module_option()
<https://docs.ceph.com/en/quincy/mgr/modules/#mgr_module.MgrModule.set_module_option>
to
set MODULE_OPTIONS values via CLI commands.
- set_store()
<https://docs.ceph.com/en/quincy/mgr/modules/#mgr_module.MgrModule.set_store>:
there are 2 main storage use cases here:
- *Immutable/sensitive data*: instead of exposing those as
MODULE_OPTIONS (password hashes, private certificates, API keys, etc.),
- *Changing data*: mgr-module internal state. While this shouldn't
cause the db to grow in the long term, it might cause short-term/compaction
issues (I'm not familiar with rocksdb internals, just extrapolating from
experience with sstable/leveldb)
For the latter case there, Dashboard developers have been looking for an
efficient alternative to persistently store rapidly-changing data. We
discarded the idea of using a pool since the Dashboard should be able to
operate prior to any OSD provisioning and in case of storage downtimes
Coming back to your original questions, I understand that there are two
different issues at stake:
- *Cluster log processing*: currently mon via Paxos (Do we really need
Paxos ack for logs? Can we live with some type of
eventually-consistent/best-effort storage here?)
- *Cluster log storage*: currently mon store db. AFAIK this is the
main issue, right?
From there, I see 2 possible paths:
- *Keep cluster-wide logs as a Ceph concern:*
- IMHO putting some throttling in place should be a must, since
client-triggered cluster logs could easily become a DoS vector.
- I wouldn't put them into a rados pool, not so much for the data
availability in case of OSD service downtime (logs will still
be recoverable from logfiles), but as for the potential interference with
user workloads/deployment patterns (as Frank mentioned before).
- Could we run the ".mgr" pool on a new type of
"internal/service-only" colocated OSDs (memstore)?
- Save logs to a fixed-size/TTL-bound priority or multi-level queue
structure?
- Add some (eventually-consistent) store db to the ceph-mgr?
- To solve ceph-mgr scalability issues, we recently added a new
kind of Ceph utility daemon (ceph-exporter) whose sole purpose is to fetch
metrics from co-located Ceph daemon's perf-counters and make those
available for Prometheus scraping. We could think about a similar thing but
for logs... (although it'd be very similar to the Loki approach below).
- *Move them outside Ceph:*
- Cephadm + Dashboard now support Centralized Logging via Loki +
Promtail <https://ceph.io/en/news/blog/2022/centralized_logging/>,
which basically polls all daemon logfiles and sends new log traces to a
central service (Loki) where they can be monitored/filtered in real-time.
- If we find the previous solution too bulky for regular cluster
monitoring, we could explore systemd-journal-remote
<https://www.freedesktop.org/software/systemd/man/systemd-journal-remote.service.html>
/rsyslog/...
- The main downside of this approach is that it might break the
"ceph log" command (rados_monitor_log and log events could still be
watched
I guess).
Kind Regards,
Ernesto
On Wed, Mar 22, 2023 at 11:12 AM Janne Johansson <icepic.dz(a)gmail.com>
wrote:
2) .mgr
pool
2.1) I have become really tired of these administrative pools that are
created on
the fly without any regards to device classes, available
capacity, PG allocation and the like. The first one that showed up without
warning was device_health_metrics, which turned the cluster health_err
right away because the on-the-fly pool creation is, well, not exactly smart.
We don't even have drives below the default root. We have a lot of
different
pools on different (custom!) device classes with different
replication schemes to accommodate a large variety of use cases.
Administrative pools showing up randomly somewhere in the tree are a real
pain. There are ceph-user cases where people deleted and recreated it only
to make the device health module useless, because it seems to store the
pool ID and there is no way to tell it to use the new pool.
Ah, that's why it looked unused after I also had to remake it. Since
it gets created when you don't have the OSDs yet, the possibilities
for it ending up wrong seem very large.
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EMEA Storage Specialistmatthias.muench(a)redhat.com
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