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From Jens Rantil <jens.ran...@tink.se>
Subject Re: When are hints written?
Date Wed, 20 Apr 2016 15:09:05 GMT
Hi Bo,

> In our case, I would like for the cluster to wait for the write to be
persisted on the relevant nodes before returning an ok to the client. But I
don't know which knobs to turn to accomplish this? or if it is even
possible :)

This is what write consistency option is for. Have a look at
https://docs.datastax.com/en/cassandra/2.0/cassandra/dml/dml_config_consistency_c.html.
Note, however that if you use ALL, your clients will fail (throw exception,
depending on language) as soon as a single partition can't be written. This
means you can't do online maintenance of a Cassandra node (such as
upgrading it etc.) without experiencing write issues.

Cheers,
Jens

On Wed, Apr 20, 2016 at 3:39 PM Bo Finnerup Madsen <bo.gundersen@gmail.com>
wrote:

> Hi,
>
> We have a small 5 node cluster of m4.xlarge clients that receives writes
> from ~20 clients. The clients will write as fast as they can, and the whole
> process is limited by the write performance of the cassandra cluster.
> After we have tweaked our schema to avoid large partitions, the load is
> going ok and we don't see any warnings or errors in the cassandra logs. But
> we do see quite a lot of hint handoff activity. During the load, the
> cassandra nodes are quite loaded, with linux reporting a load as high as 20.
>
> I have read the available documentation on how hints works, and to my
> understanding hints should only be written if a node is down. But as far as
> I can see, none of the nodes are marked as down during the load. So I
> suspect I am missing something :)
> We have configured the servers with write_request_timeout_in_ms: 120000
> and the clients with a timeout of 130000, but still get hints stored.
>
> In our case, I would like for the cluster to wait for the write to be
> persisted on the relevant nodes before returning an ok to the client. But I
> don't know which knobs to turn to accomplish this? or if it is even
> possible :)
>
> We are running cassandra 3.0.3, with 8Gb heap and a replication factor of
> 3.
>
> Thank you in advance!
>
> Yours sincerely,
>   Bo Madsen
>
-- 

Jens Rantil
Backend Developer @ Tink

Tink AB, Wallingatan 5, 111 60 Stockholm, Sweden
For urgent matters you can reach me at +46-708-84 18 32.

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