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TWCS: Repair create new buckets with old data


Hi Maik,

Yes when u have old and new data mixed together the old SStable will not be dropped until new SStable is fully expired.

There are couple of ways for you to reclaim the storage,

1.) If this is one time thing probably, you can manually run some commands which will rewrite sstables like, nodetool compact, scrub or garbagecollect   

2.) If u think this would be recurring probably u should set unchecked_tombstone_compaction to true default is false
unchecked_tombstone_compaction (default: false)
The single sstable compaction has quite strict checks for whether it should be started, this option disables those checks and for some use cases this might be needed. Note that this does not change anything for the actual compaction, tombstones are only dropped if it is safe to do so - it might just rewrite an sstable without being able to drop any tombstones.

In both cases it will trigger numerous compactions make sure u have enough i/o or throttle ur compaction threads 

If u r going to make DDL change - option 2, would recommend u to go through some more info on how TWCS wrks   



Thanks
Sri Rathan

Hallo,

we work with Cassandra version 3.0.9 and have a problem in a table with TWCS. The command “nodetool repair” create always new files with old data. This avoid the delete of the old data.

The layout of the Table is following:

cqlsh> desc stat.spa

 

CREATE TABLE stat.spa (

    region int,

    id int,

    date text,

    hour int,

    zippedjsonstring blob,

    PRIMARY KEY ((region, id), date, hour)

) WITH CLUSTERING ORDER BY (date ASC, hour ASC)

    AND bloom_filter_fp_chance = 0.01

    AND caching = {'keys': 'ALL', 'rows_per_partition': 'NONE'}

    AND comment = ''

    AND compaction = {'class': 'org.apache.cassandra.db.compaction.TimeWindowCompactionStrategy', 'compaction_window_size': '1', 'compaction_window_unit': 'DAYS', 'max_threshold': '100', 'min_threshold': '4', 'tombstone_compaction_interval': '86460'}

    AND compression = {'chunk_length_in_kb': '64', 'class': 'org.apache.cassandra.io.compress.LZ4Compressor'}

    AND crc_check_chance = 1.0

    AND dclocal_read_repair_chance = 0.0

    AND default_time_to_live = 0

    AND gc_grace_seconds = 864000

    AND max_index_interval = 2048

    AND memtable_flush_period_in_ms = 0

    AND min_index_interval = 128

    AND read_repair_chance = 0.0

    AND speculative_retry = '99PERCENTILE';

 

Actual the oldest data are from 2017/04/15 and will not remove:

 

$ for f in *Data.db; do meta=$(sudo sstablemetadata $f); echo -e "Max:" $(date --date=@$(echo "$meta" | grep Maximum\ time | cut -d" "  -f3| cut -c 1-10) '+%Y/%m/%d %H:%M') "Min:" $(date --date=@$(echo "$meta" | grep Minimum\ time | cut -d" "  -f3| cut -c 1-10) '+%Y/%m/%d %H:%M') $(echo "$meta" | grep droppable) $(echo "$meta" | grep "Repaired at") ' \t ' $(ls -lh $f | awk '{print $5" "$6" "$7" "$8" "$9}'); done | sort

Max: 2017/04/15 12:08 Min: 2017/03/31 13:09 Estimated droppable tombstones: 1.7731048805815162 Repaired at: 1525685601400         42K May 7 19:56 mc-22922-big-Data.db

Max: 2017/04/17 13:49 Min: 2017/03/31 13:09 Estimated droppable tombstones: 1.9600207684319835 Repaired at: 1525685601400         116M May 7 13:31 mc-15096-big-Data.db

Max: 2017/04/21 13:43 Min: 2017/04/15 13:34 Estimated droppable tombstones: 1.9090909090909092 Repaired at: 1525685601400         11K May 7 19:56 mc-22921-big-Data.db

Max: 2017/05/23 21:45 Min: 2017/04/21 14:00 Estimated droppable tombstones: 1.8360655737704918 Repaired at: 1525685601400         21M May 7 19:56 mc-22919-big-Data.db

Max: 2017/06/12 15:19 Min: 2017/04/25 14:45 Estimated droppable tombstones: 1.8091397849462365 Repaired at: 1525685601400         19M May 7 14:36 mc-17095-big-Data.db

Max: 2017/06/15 15:26 Min: 2017/05/10 14:37 Estimated droppable tombstones: 1.76536312849162 Repaired at: 1529612605539           9.3M Jun 21 22:31 mc-25372-big-Data.db

 

After a „nodetool repair“ run, a new big data file is created that include old data from 2017/07/31.

 

Max: 2018/07/27 18:10 Min: 2017/03/31 13:13 Estimated droppable tombstones: 0.08392555471691247 Repaired at: 0            11G Sep 11 22:02 mc-39281-big-Data.db

Max: 2018/08/16 18:18 Min: 2018/08/06 12:19 Estimated droppable tombstones: 0.0 Repaired at: 1534525730510        123M Aug 17 23:46 mc-36847-big-Data.db

Max: 2018/08/17 19:20 Min: 2017/07/31 12:04 Estimated droppable tombstones: 0.03385963490004347 Repaired at: 0            11G Sep 11 21:43 mc-39265-big-Data.db

Max: 2018/08/17 19:20 Min: 2018/07/24 12:33 Estimated droppable tombstones: 0.0 Repaired at: 1534525730510        135M Sep 11 21:44 mc-39270-big-Data.db

Max: 2018/09/06 17:30 Min: 2018/08/28 12:17 Estimated droppable tombstones: 0.0 Repaired at: 1536690786879        129M Sep 11 21:10 mc-39238-big-Data.db

Max: 2018/09/07 18:22 Min: 2017/04/23 12:48 Estimated droppable tombstones: 0.1548442441468401 Repaired at: 0     8.0G Sep 11 21:33 mc-39258-big-Data.db

Max: 2018/09/07 18:22 Min: 2018/09/07 12:15 Estimated droppable tombstones: 0.0 Repaired at: 1536690786879        72M Sep 11 21:34 mc-39262-big-Data.db

Max: 2018/09/08 18:20 Min: 2018/08/22 12:17 Estimated droppable tombstones: 0.0 Repaired at: 0            2.8G Sep 11 21:47 mc-39272-big-Data.db

 

The tool sstableexpiredblockers shows that the file mc-39281-big-Data.db blocks 95 expired files from getting dropped, for example the oldest file mc-22922-big-Data.db

 

[BigTableReader(path='.../stat/spa-.../mc-39281-big-Data.db') (minTS = 1490958782530000, maxTS = 1532707837676719, maxLDT = 1557154990)

  blocks 95 expired sstables from getting dropped:

 [BigTableReader(path='.../stat/spa-.../mc-36936-big-Data.db') (minTS = 1500027128958000, maxTS = 1503666765807229, maxLDT = 1535202765)

[BigTableReader(path='.../stat/spa-.../mc-22921-big-Data.db') (minTS = 1492256093314000, maxTS = 1492775013454001, maxLDT = 1524311013)

[BigTableReader(path='.../stat/spa-.../mc-36947-big-Data.db') (minTS = 1492255708403000, maxTS = 1501937182477001, maxLDT = 1533473182)

[BigTableReader(path='.../stat/spa-.../mc-32582-big-Data.db') (minTS = 1493028031639000, maxTS = 1499175057476001, maxLDT = 1530711057)

[BigTableReader(path='.../stat/spa-.../mc-32560-big-Data.db') (minTS = 1500210297826000, maxTS = 1501416691390001, maxLDT = 1532952691)

[BigTableReader(path='.../stat/spa-.../mc-32528-big-Data.db') (minTS = 1490958761762000, maxTS = 1504358072394248, maxLDT = 1535894072)

[BigTableReader(path='.../stat/spa-.../mc-32572-big-Data.db') (minTS = 1500027103795000, maxTS = 1500297137808001, maxLDT = 1531833137)

[BigTableReader(path='.../stat/spa-.../mc-36935-big-Data.db') (minTS = 1500038582669000, maxTS = 1503839159485824, maxLDT = 1535375159)

[BigTableReader(path='.../stat/spa-.../mc-22922-big-Data.db') (minTS = 1490958570018000, maxTS = 1492250905633001, maxLDT = 1523786905)

[BigTableReader(path='.../stat/spa-.../mc-33470-big-Data.db') (minTS = 1499940836241000, maxTS = 1500040376685000, maxLDT = 1531576376)

 

Why create the repair such turbulence in new data files and how can we remove the old data?

 

Kind Regards

Maik Cäsar

 



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