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From Benedict Elliott Smith <>
Subject Re: Guidelines for configuring Thresholds for Cassandra metrics
Date Thu, 08 Sep 2016 12:38:15 GMT
The thing is, this isn't about opinions.  I don't really want to get into
an argument either, but characterising my statements as opinion does invite
me to respond with their factual basis...  The only legitimate opinions
here would be around the prevalence of cluster characteristics, of which we
both have anecdata (and of which your anecdata is no doubt a larger sample
than mine, but probably no less biased a sample).

To start, I will note I have already indicated that I believe a majority of
clusters are likely to become compaction-constrained, and as a result the
defaults may not be optimal.  However:

1) There are definitely workloads that are not compaction constrained, and
these clusters definitely do not want the values you suggest.  Cassandra is
best used for bursty workloads, where a cluster is briefly saturated and
has periods of relative calm.  In these workloads, there is plenty of time
to make up compaction later, and is really the whole benefit of LSMTs that
C* offers.  In these systems, a lower value is definitely preferable.

2) Similarly, in clusters with DTCS/TWCS, heavy-TTL use, or simply very
small datasets, compaction costs are significantly curtailed, and a lower
value can easily be preferred to increase cluster throughput.

3) Finally, if you have one data table that receives a majority of your
writes, a value of 0.6 will be liable to cause write stalls because only
one memtable can be flushing at any moment.  Once you have more than one
data table receiving a *significant* proportion of writes this becomes a
non-issue, but since Cassandra is optimal with only one main data table,
and fewer tables are encouraged, this is a *really important point for a
subset of users*.  These users should *definitely* not use your suggested
value, and should cap at 0.49 or less.

I will reiterate for the umpteenth time I'm not contradicting your
statements that the defaults can be improved or that your suggestion is
better for (probably) the majority of users.  But it is *not* a *universal*
truth, and the blanket statement may encourage a proliferation of bad

But I will also reiterate the documented advice in the yaml is indeed
atrocious, and offers much more misleading advice than yours, so we should
perhaps try to distill this discussion into a more palatable snippet for
the yaml.

On 29 August 2016 at 16:22, Ryan Svihla <> wrote:

> Benedict I really don't want to turn this into a battle about who's
> opinion is more valid and I really respect all the good work you've done
> for Apache Cassandra.
> I'll just reiterate that I'm comfortable saying 0.6 is a good starting
> point and it is often not the ideal once you go through more thorough
> testing, all of which I said initially and I still think is a reasonable
> statement.
> -regards,
> Ryan Svihla
> On Sat, Aug 27, 2016 at 9:31 AM -0500, "Benedict Elliott Smith" <
>> wrote:
> I did not claim you had no evidence, only that your statement lacked
>> justification.  Again, nuance is important.
>> I was suggesting that blanket statements without the necessary caveats,
>> to the user mailing list, countermanding the defaults without
>> 'justification' (explanation, reasoning) is liable to cause confusion on
>> what best practice is.  I attempted to provide some of the missing context
>> to minimise this confusion while still largely agreeing with you.
>> However you should also bear in mind that you work as a field engineer
>> for DataStax, and as such your sample of installation behaviours will be
>> biased - towards those where the defaults have not worked well.
>> On Saturday, 27 August 2016, Ryan Svihla <> wrote:
>>>  I have been trying to get the docs fixed for this for the past 3
>>> months, and there already is a ticket open for changing the defaults. I
>>> don't feel like I've had a small amount of evidence here. All observation
>>> in the 3 years of work in the field suggests compaction keeps coming up as
>>> the bottleneck when you push Cassandra ingest.
>>> 0.6 as an initial setting has fixed 20+ broken clusters in practice and
>>> it improved overall performance in every case from defaults of 0.33 to
>>> defaults of 0.03 (yaml suggests per core flush writers, add in the
>>> prevelance of HT and you see a lot of 24+ flush writer systems in the wild)
>>> No disrespect intended but that default hasn't worked out well at all in
>>> my exposure to it, and 0.6 has never been worse than the default yet.
>>> Obviously write patterns, heap configuration, memtable size limits and what
>>> not affect the exact optimal setting and I've rarely had it end up 0.6
>>> after a tuning exercise. I never intended that as a blanket recommendation,
>>> just a starting one.
>>> _____________________________
>>> From: Benedict Elliott Smith <>
>>> Sent: Friday, August 26, 2016 9:40 AM
>>> Subject: Re: Guidelines for configuring Thresholds for Cassandra metrics
>>> To: <>
>>> The default when I wrote it was 0.4 but it was found this did not
>>> saturate flush writers in JBOD configurations. Iirc it now defaults to
>>> 1/(1+#disks) which is not a terrible default, but obviously comes out much
>>> lower if you have many disks.
>>> This smaller value behaves better for peak performance, but in a live
>>> system where compaction is king not saturating flush in return for lower
>>> write amplification (from flushing larger memtables) will indeed often be a
>>> win.
>>> 0.6, however, is probably not the best default unless you have a lot of
>>> tables being actively written to, in which case even 0.8 would be fine.
>>> With a single main table receiving your writes at a given time, 0.4 is
>>> probably an optimal value, when making this trade off against peak
>>> performance.
>>> Anyway, it's probably better to file a ticket to discuss defaults and
>>> documentation than making a statement like this without justification. I
>>> can see where you're coming from, but it's confusing for users to have such
>>> blanket guidance that counters the defaults.  If the defaults can be
>>> improved (which I agree they can) it's probably better to do that, along
>>> with better documentation, so the nuance is accounted for.
>>> On Friday, 26 August 2016, Ryan Svihla <> wrote:
>>>> Forgot the most important thing. Logs
>>>> ERROR you should investigate
>>>> WARN you should have a list of known ones. Use case dependent. Ideally
>>>> you change configuration accordingly.
>>>> *PoolCleaner (slab or native) - good indication node is tuned badly if
>>>> you see a ton of this. Set memtable_cleanup_threshold to 0.6 as an initial
>>>> attempt to configure this correctly.  This is a complex topic to dive into,
>>>> so that may not be the best number, it'll likely be better than the
>>>> default, why its not the default is a big conversation.
>>>> There are a bunch of other logs I look for that are escaping me at
>>>> present but that's a good start
>>>> -regards,
>>>> Ryan Svihla
>>>> On Fri, Aug 26, 2016 at 7:21 AM -0500, "Ryan Svihla" <>
>>>> wrote:
>>>> Thomas,
>>>>> Not all metrics are KPIs and are only useful when researching a
>>>>> specific issue or after a use case specific threshold has been set.
>>>>> The main "canaries" I monitor are:
>>>>> * Pending compactions (dependent on the compaction strategy chosen but
>>>>> 1000 is a sign of severe issues in all cases)
>>>>> * dropped mutations (more than one I treat as a event to investigate,
>>>>> I believe in allowing operational overhead and any evidence of load
>>>>> shedding suggests I may not have as much as I thought)
>>>>> * blocked anything (flush writers, etc..more than one I investigate)
>>>>> * system hints ( More than 1k I investigate)
>>>>> * heap usage and gc time vary a lot by use case and collector chosen,
>>>>> I aim for below 65% usage as an average with g1, but this again varies
>>>>> use case a great deal. Sometimes I just looks the chart and query patterns
>>>>> and if they don't line up I have to do other deeper investigations
>>>>> * read and write latencies exceeding SLA is also use case dependent.
>>>>> Those that have none I tend to push towards p99 with a middle end SSD
>>>>> system having 100ms and a spindle based system having 600ms with CL one
>>>>> assuming a "typical" query pattern (again query patterns and CL so vary
>>>>> here)
>>>>> * cell count and partition size vary greatly by hardware and gc tuning
>>>>> but I like to in the absence of all other relevant information like to
>>>>> cell count for a partition below 100k and size below 100mb. I however
>>>>> many successful use cases running more and I've had some fail well before
>>>>> that. Hardware and tuning tradeoff a shift this around a lot.
>>>>> There is unfortunately as you'll note a lot of nuance and the load out
>>>>> really changes what looks right (down to the model of SSDs I have different
>>>>> expectations for p99s if it's a model I haven't used before I'll do some
>>>>> comparative testing).
>>>>> The reason so much of this is general and vague is my selection bias.
>>>>> I'm brought in when people are complaining about performance or some
>>>>> systemic crash because they were monitoring nothing. I have little ability
>>>>> to change hardware initially so I have to be willing to allow the hardware
>>>>> to do the best it can an establish levels where it can no longer keep
>>>>> with the customers goals. This may mean for some use cases 10 pending
>>>>> compactions is an actionable event for them, for another customer 100
>>>>> The better approach is to establish a baseline for when these metrics
>>>>> to indicate a serious issue is occurring in that particular app. Basically
>>>>> when people notice a problem, what did these numbers look like in the
>>>>> minutes, hours and days prior? That's the way to establish the levels
>>>>> consistently.
>>>>> Regards,
>>>>> Ryan Svihla
>>>>> On Fri, Aug 26, 2016 at 4:48 AM -0500, "Thomas Julian" <
>>>>>> wrote:
>>>>> Hello,
>>>>>> I am working on setting up a monitoring tool to monitor Cassandra
>>>>>> Instances. Are there any wikis which specifies optimum value for
>>>>>> Cassandra KPIs?
>>>>>> For instance, I am not sure,
>>>>>>    1. What value of "Memtable Columns Count" can be considered as
>>>>>>    "Normal".
>>>>>>    2. What value of the same has to be considered as "Critical".
>>>>>> I knew threshold numbers for few params, for instance any thing more
>>>>>> than zero for timeouts, pending tasks should be considered as
>>>>>> unusual. Also, I am aware that most of the statistics' threshold
>>>>>> vary in accordance with Hardware Specification, Cassandra Environment
>>>>>> Setup. But, what I request here is a general guideline for configuring
>>>>>> thresholds for all the metrics.
>>>>>> If this has been already covered, please point me to that resource.
>>>>>> If anyone on their own interest collected these things, please share.
>>>>>> Any help is appreciated.
>>>>>> Best Regards,
>>>>>> Julian.

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