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From Shalom Sagges <shal...@liveperson.com>
Subject Re: Some questions to updating and tombstone
Date Wed, 16 Nov 2016 09:15:37 GMT
Hi Fabrice,

Just a small (out of the topic) question I couldn't find an answer to. What
is a slice in Cassandra? (e.g. Maximum tombstones per slice)

Thanks!


Shalom Sagges
DBA
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On Tue, Nov 15, 2016 at 6:38 PM, Fabrice Facorat <fabrice.facorat@gmail.com>
wrote:

> If you don't want tombstones, don't generate them ;)
>
> More seriously, tombstones are generated when:
> - doing a DELETE
> - TTL expiration
> - set a column to NULL
>
> However tombstones are an issue only if for the same value, you have many
> tombstones (i.e you keep overwriting the same values with datas and
> tombstones). Having 1 tombstone for 1 value is not an issue, having 1000
> tombstone for 1 value is a problem. Do really your use case overwrite data
> with DELETE or  NULL ?
>
> So that's why what you may want to know is how many tombstones you have on
> average when reading a value. This is available in:
> - nodetool cfstats ks.cf : Average tombstones per slice/Maximum
> tombstones per slice
> - JMX : org.apache.cassandra.metrics:keyspace=<ks>,name=
> TombstoneScannedHistogram,scope=<cf>,type=ColumnFamily
> Max/Count/99thPercentile/Mean
>
>
> 2016-11-15 10:05 GMT+01:00 Lu, Boying <Boying.Lu@dell.com>:
>
>> Thanks a lot for your help.
>>
>>
>>
>> We are using STCS strategy and not using TTL
>>
>>
>>
>> Is there any API that we can use to query the current number of
>> tombstones in a CF?
>>
>>
>>
>>
>>
>>
>>
>> *From:* Anuj Wadehra [mailto:anujw_2003@yahoo.co.in]
>> *Sent:* 2016年11月14日 22:20
>> *To:* user@cassandra.apache.org
>> *Subject:* Re: Some questions to updating and tombstone
>>
>>
>>
>> Hi Boying,
>>
>>
>>
>> I agree with Vladimir.If compaction is not compacting the two sstables
>> with updates soon, disk space issues will be wasted. For example, if the
>> updates are not closer in time, first update might be in a big table by the
>> time second update is being written in a new small table. STCS wont compact
>> them together soon.
>>
>>
>>
>> Just adding column values with new timestamp shouldnt create any
>> tombstones. But if data is not merged for long, disk space issues may
>> arise. If you are STCS,just  yo get an idea about the extent of the problem
>> you can run major compaction and see the amount of disk space created with
>> that( dont do this in production as major compaction has its own side
>> effects).
>>
>>
>>
>> Which compaction strategy are you using?
>>
>> Are these updates done with TTL?
>>
>>
>>
>> Thanks
>> Anuj
>>
>>
>>
>> On Mon, 14 Nov, 2016 at 1:54 PM, Vladimir Yudovin
>>
>> <vladyu@winguzone.com> wrote:
>>
>> Hi Boying,
>>
>>
>>
>> UPDATE write new value with new time stamp. Old value is not tombstone,
>> but remains until compaction. gc_grace_period is not related to this.
>>
>>
>>
>> Best regards, Vladimir Yudovin,
>>
>>
>> *Winguzone <https://winguzone.com?from=list> - Hosted Cloud Cassandra
>> Launch your cluster in minutes.*
>>
>>
>>
>>
>>
>> ---- On Mon, 14 Nov 2016 03:02:21 -0500*Lu, Boying <Boying.Lu@dell.com
>> <Boying.Lu@dell.com>>* wrote ----
>>
>>
>>
>> Hi, All,
>>
>>
>>
>> Will the Cassandra generates a new tombstone when updating a column by
>> using CQL update statement?
>>
>>
>>
>> And is there any way to get the number of tombstones of a column family
>> since we want to void generating
>>
>> too many tombstones within gc_grace_period?
>>
>>
>>
>> Thanks
>>
>>
>>
>> Boying
>>
>>
>>
>>
>
>
> --
> Close the World, Open the Net
> http://www.linux-wizard.net
>

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