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From Alex Kotelnikov <alex.kotelni...@diginetica.com>
Subject Re: Full table scan with cassandra
Date Thu, 17 Aug 2017 16:36:54 GMT
Dor,

I believe, I tried it in many ways and the result is quite disappointing.
I've run my scans on 3 different clusters, one of which was using on VMs
and I was able to scale it up and down (3-5-7 VMs, 8 to 24 cores) to see,
how this affects the performance.

I also generated the flow from spark cluster ranging from 4 to 40 parallel
tasks as well as just multi-threaded client.

The surprise is that trivial fetch of all records using token ranges takes
pretty much the same time in all setups.

The only beneficial thing I've learned is that it is much more efficient to
create a MATERIALIZED VIEW than to filter (even using secondary index).

Say, I have a typical dataset, around 3Gb of data, 1M records. And I have a
trivial scan practice:

String.format("SELECT token(user_id), user_id, events FROM user_events
WHERE token(user_id) >= %d ", start) + (end != null ? String.format(" AND
token(user_id) < %d ", end) : "")

I split all tokens into start-end ranges (except for last range, which only
has start) and query ranges in multiple threads, up to 40.

Whole process takes ~40s on 3 VMs cluster  2+2+4 cores, 16Gb RAM each 1
virtual disk. And it takes ~30s on real hardware clusters
8servers*8cores*32Gb. Level of the concurrency does not matter pretty much
at all. Util it is too high or too low.
Size of tokens range matters, but here I see the rule "make it larger, but
avoid cassandra timeouts".
I also tried spark connector to validate that my test multithreaded app is
not the bottleneck. It is not.

I expected some kind of elasticity, I see none. Feels like I do something
wrong...



On 17 August 2017 at 00:19, Dor Laor <dor@scylladb.com> wrote:

> Hi Alex,
>
> You probably didn't get the paralelism right. Serial scan has
> a paralelism of one. If the paralelism isn't large enough, perf will be
> slow.
> If paralelism is too large, Cassandra and the disk will trash and have too
> many context switches.
>
> So you need to find your cluster's sweet spot. We documented the procedure
> to do it in this blog: http://www.scylladb.com/
> 2017/02/13/efficient-full-table-scans-with-scylla-1-6/
> and the results are here: http://www.scylladb.com/
> 2017/03/28/parallel-efficient-full-table-scan-scylla/
> The algorithm should translate to Cassandra but you'll have to use
> different rules of the thumb.
>
> Best,
> Dor
>
>
> On Wed, Aug 16, 2017 at 9:50 AM, Alex Kotelnikov <
> alex.kotelnikov@diginetica.com> wrote:
>
>> Hey,
>>
>> we are trying Cassandra as an alternative for storage huge stream of data
>> coming from our customers.
>>
>> Storing works quite fine, and I started to validate how retrieval does.
>> We have two types of that: fetching specific records and bulk retrieval for
>> general analysis.
>> Fetching single record works like charm. But it is not so with bulk fetch.
>>
>> With a moderately small table of ~2 million records, ~10Gb raw data I
>> observed very slow operation (using token(partition key) ranges). It takes
>> minutes to perform full retrieval. We tried a couple of configurations
>> using virtual machines, real hardware and overall looks like it is not
>> possible to all table data in a reasonable time (by reasonable I mean that
>> since we have 1Gbit network 10Gb can be transferred in a couple of minutes
>> from one server to another and when we have 10+ cassandra servers and 10+
>> spark executors total time should be even smaller).
>>
>> I tried datastax spark connector. Also I wrote a simple test case using
>> datastax java driver and see how fetch of 10k records takes ~10s so I
>> assume that "sequential" scan will take 200x more time, equals ~30 minutes.
>>
>> May be we are totally wrong trying to use Cassandra this way?
>>
>> --
>>
>> Best Regards,
>>
>>
>> *Alexander Kotelnikov*
>>
>> *Team Lead*
>>
>> DIGINETICA
>> Retail Technology Company
>>
>> m: +7.921.915.06.28 <+7%20921%20915-06-28>
>>
>> *www.diginetica.com <http://www.diginetica.com/>*
>>
>
>


-- 

Best Regards,


*Alexander Kotelnikov*

*Team Lead*

DIGINETICA
Retail Technology Company

m: +7.921.915.06.28

*www.diginetica.com <http://www.diginetica.com/>*

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