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What does rows_merged mean in compactionhistory?
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Solved cassandradatastax-enterprisedatastax
A

1

8

When I issue

$ nodetool compactionhistory

I get

. . . compacted_at        bytes_in       bytes_out      rows_merged
. . . 1404936947592       8096           7211           {1:3, 3:1}

What does {1:3, 3:1} mean? The only documentation I can find is this which states

the number of partitions merged

which does not explain why multiple values and what the colon means.

Amblyopia answered 19/12, 2014 at 12:49 Comment(0)
I
25

So basically it means {tables:rows} for example {1:3, 3:1} means 3 rows were taken from one sstable (1:3) and 1 row taken from 3 (3:1) sstables, all to make the one sstable in that compaction operation.

I tried it out myself so here's an example, I hope this helps:

create keyspace and table:

cqlsh> create keyspace space1 WITH replication = {'class': 'SimpleStrategy', 'replication_factor': 1};

cqlsh> create TABLE space1.tb1 ( key text, val1 text, primary KEY (key));

cqlsh> INSERT INTO space1.tb1 (key, val1 ) VALUES ( 'key1','111');
cqlsh> INSERT INTO space1.tb1 (key, val1 ) VALUES ( 'key2','222');
cqlsh> INSERT INTO space1.tb1 (key, val1 ) VALUES ( 'key3','333');
cqlsh> INSERT INTO space1.tb1 (key, val1 ) VALUES ( 'key4','444');
cqlsh> INSERT INTO space1.tb1 (key, val1 ) VALUES ( 'key5','555');
cqlsh> exit

Now we flush to create the sstable

$ nodetool flush space1

We see that only one version of the table is created

$ sudo ls -lR /var/lib/cassandra/data/space1

/var/lib/cassandra/data/space1:
total 4
drwxr-xr-x. 2 cassandra cassandra 4096 Feb  3 12:51 tb1

/var/lib/cassandra/data/space1/tb1:
total 32
-rw-r--r--. 1 cassandra cassandra   43 Feb  3 12:51 space1-tb1-jb-1-CompressionInfo.db
-rw-r--r--. 1 cassandra cassandra  146 Feb  3 12:51 space1-tb1-jb-1-Data.db
-rw-r--r--. 1 cassandra cassandra   24 Feb  3 12:51 space1-tb1-jb-1-Filter.db
-rw-r--r--. 1 cassandra cassandra   90 Feb  3 12:51 space1-tb1-jb-1-Index.db
-rw-r--r--. 1 cassandra cassandra 4389 Feb  3 12:51 space1-tb1-jb-1-Statistics.db
-rw-r--r--. 1 cassandra cassandra   80 Feb  3 12:51 space1-tb1-jb-1-Summary.db
-rw-r--r--. 1 cassandra cassandra   79 Feb  3 12:51 space1-tb1-jb-1-TOC.txt

check the sstable2json we see our data

$ sudo -u cassandra /usr/bin/sstable2json /var/lib/cassandra/data/space1/tb1/space1-tb1-jb-1-Data.db
[
{"key": "6b657935","columns": [["","",1422967847005000], ["val1","555",1422967847005000]]},
{"key": "6b657931","columns": [["","",1422967817740000], ["val1","111",1422967817740000]]},
{"key": "6b657934","columns": [["","",1422967840622000], ["val1","444",1422967840622000]]},
{"key": "6b657933","columns": [["","",1422967832341000], ["val1","333",1422967832341000]]},
{"key": "6b657932","columns": [["","",1422967825116000], ["val1","222",1422967825116000]]}
]

At this point ‘notetool compactionhistory’ shows nothing for this table but lets run compact anyway to see what we get (scroll right)

$ nodetool compactionhistory | awk 'NR == 2 || /space1/'
id                                       keyspace_name      columnfamily_name            compacted_at              bytes_in       bytes_out      rows_merged
5725f890-aba4-11e4-9f73-351725b0ac5b     space1             tb1                          1422968305305             146            146            {1:5}

Now lets delete two rows, and flush

cqlsh> delete from space1.tb1 where key='key1';
cqlsh> delete from space1.tb1 where key='key2';
cqlsh> exit

$ nodetool flush space1

$ sudo ls -l /var/lib/cassandra/data/space1/tb1/
[sudo] password for datastax: 
total 64
-rw-r--r--. 1 cassandra cassandra   43 Feb  3 12:58 space1-tb1-jb-2-CompressionInfo.db
-rw-r--r--. 1 cassandra cassandra  146 Feb  3 12:58 space1-tb1-jb-2-Data.db
-rw-r--r--. 1 cassandra cassandra  336 Feb  3 12:58 space1-tb1-jb-2-Filter.db
-rw-r--r--. 1 cassandra cassandra   90 Feb  3 12:58 space1-tb1-jb-2-Index.db
-rw-r--r--. 1 cassandra cassandra 4393 Feb  3 12:58 space1-tb1-jb-2-Statistics.db
-rw-r--r--. 1 cassandra cassandra   80 Feb  3 12:58 space1-tb1-jb-2-Summary.db
-rw-r--r--. 1 cassandra cassandra   79 Feb  3 12:58 space1-tb1-jb-2-TOC.txt
-rw-r--r--. 1 cassandra cassandra   43 Feb  3 13:02 space1-tb1-jb-3-CompressionInfo.db
-rw-r--r--. 1 cassandra cassandra   49 Feb  3 13:02 space1-tb1-jb-3-Data.db
-rw-r--r--. 1 cassandra cassandra   16 Feb  3 13:02 space1-tb1-jb-3-Filter.db
-rw-r--r--. 1 cassandra cassandra   36 Feb  3 13:02 space1-tb1-jb-3-Index.db
-rw-r--r--. 1 cassandra cassandra 4413 Feb  3 13:02 space1-tb1-jb-3-Statistics.db
-rw-r--r--. 1 cassandra cassandra   80 Feb  3 13:02 space1-tb1-jb-3-Summary.db
-rw-r--r--. 1 cassandra cassandra   79 Feb  3 13:02 space1-tb1-jb-3-TOC.txt

Lets check the tables contents

$ sudo -u cassandra /usr/bin/sstable2json /var/lib/cassandra/data/space1/tb1/space1-tb1-jb-2-Data.db
[
{"key": "6b657935","columns": [["","",1422967847005000], ["val1","555",1422967847005000]]},
{"key": "6b657931","columns": [["","",1422967817740000], ["val1","111",1422967817740000]]},
{"key": "6b657934","columns": [["","",1422967840622000], ["val1","444",1422967840622000]]},
{"key": "6b657933","columns": [["","",1422967832341000], ["val1","333",1422967832341000]]},
{"key": "6b657932","columns": [["","",1422967825116000], ["val1","222",1422967825116000]]}
]

$ sudo -u cassandra /usr/bin/sstable2json /var/lib/cassandra/data/space1/tb1/space1-tb1-jb-3-Data.db
[
{"key": "6b657931","metadata": {"deletionInfo": {"markedForDeleteAt":1422968551313000,"localDeletionTime":1422968551}},"columns": []},
{"key": "6b657932","metadata": {"deletionInfo": {"markedForDeleteAt":1422968553322000,"localDeletionTime":1422968553}},"columns": []}
]

Now lets compact 

$ nodetool compact space1

Only one stable now as expected

$ sudo ls -l /var/lib/cassandra/data/space1/tb1/
total 32
-rw-r--r--. 1 cassandra cassandra   43 Feb  3 13:05 space1-tb1-jb-4-CompressionInfo.db
-rw-r--r--. 1 cassandra cassandra  133 Feb  3 13:05 space1-tb1-jb-4-Data.db
-rw-r--r--. 1 cassandra cassandra  656 Feb  3 13:05 space1-tb1-jb-4-Filter.db
-rw-r--r--. 1 cassandra cassandra   90 Feb  3 13:05 space1-tb1-jb-4-Index.db
-rw-r--r--. 1 cassandra cassandra 4429 Feb  3 13:05 space1-tb1-jb-4-Statistics.db
-rw-r--r--. 1 cassandra cassandra   80 Feb  3 13:05 space1-tb1-jb-4-Summary.db
-rw-r--r--. 1 cassandra cassandra   79 Feb  3 13:05 space1-tb1-jb-4-TOC.txt

Now lets check the contents of the new stable we can see the tombstones

$ sudo -u cassandra /usr/bin/sstable2json /var/lib/cassandra/data/space1/tb1/space1-tb1-jb-4-Data.db
[
{"key": "6b657935","columns": [["","",1422967847005000], ["val1","555",1422967847005000]]},
{"key": "6b657931","metadata": {"deletionInfo": {"markedForDeleteAt":1422968551313000,"localDeletionTime":1422968551}},"columns": []},
{"key": "6b657934","columns": [["","",1422967840622000], ["val1","444",1422967840622000]]},
{"key": "6b657933","columns": [["","",1422967832341000], ["val1","333",1422967832341000]]},
{"key": "6b657932","metadata": {"deletionInfo": {"markedForDeleteAt":1422968553322000,"localDeletionTime":1422968553}},"columns": []}
]

Finally lets check compaction history (scroll right)

$ nodetool compactionhistory | awk 'NR == 2 || /space1/'
id                                       keyspace_name      columnfamily_name            compacted_at              bytes_in       bytes_out      rows_merged
5725f890-aba4-11e4-9f73-351725b0ac5b     space1             tb1                          1422968305305             146            146            {1:5}
46112600-aba5-11e4-9f73-351725b0ac5b     space1             tb1                          1422968706144             195            133            {1:3, 2:2}
Inflationism answered 4/2, 2015 at 14:27 Comment(2)
Wow, this is a brilliant answer! – Lipcombe
Couldn't agree more. Thanks! – Amblyopia

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