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From "Jonathan Ellis (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-10195) TWCS experiments and improvement proposals
Date Wed, 26 Aug 2015 14:55:46 GMT


Jonathan Ellis commented on CASSANDRA-10195:

Do either of you have TWCS-vs-DTCS data demonstrating a win in either write or read amplification?
 (i.e. bytes compacted or sstables touched per read) ?

> TWCS experiments and improvement proposals
> ------------------------------------------
>                 Key: CASSANDRA-10195
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Antti Nissinen
>             Fix For: 2.1.x, 2.2.x
>         Attachments: 20150814_1027_compaction_hierarchy.txt, node0_20150727_1250_time_graph.txt,
node0_20150810_1017_time_graph.txt, node0_20150812_1531_time_graph.txt, node0_20150813_0835_time_graph.txt,
node0_20150814_1054_time_graph.txt, node1_20150727_1250_time_graph.txt, node1_20150810_1017_time_graph.txt,
node1_20150812_1531_time_graph.txt, node1_20150813_0835_time_graph.txt, node1_20150814_1054_time_graph.txt,
node2_20150727_1250_time_graph.txt, node2_20150810_1017_time_graph.txt, node2_20150812_1531_time_graph.txt,
node2_20150813_0835_time_graph.txt, node2_20150814_1054_time_graph.txt, sstable_count_figure1.png,
> This JIRA item describes experiments with DateTieredCompactionStartegy (DTCS) and TimeWindowCompactionStrategy
(TWCS) and proposes modifications to the TWCS. In a test system several crashes were caused
intentionally (and unintentionally) and repair operations were executed leading to flood of
small SSTables. Target was to be able compact those files are release disk space reserved
by duplicate data. Setup is following:
> - Three nodes
> - DateTieredCompactionStrategy, max_sstable_age_days = 5
>     Cassandra 2.1.2
> The setup and data format has been documented in detailed here
> The test was started by dumping  few days worth of data to the database for 100 000 signals.
Time graphs of SStables from different nodes indicates that the DTCS has been working as expected
and SStables are nicely ordered in time wise.
> See files:
> node0_20150727_1250_time_graph.txt
> node1_20150727_1250_time_graph.txt
> node2_20150727_1250_time_graph.txt
> Status=Up/Down
> |/ State=Normal/Leaving/Joining/Moving
> --  Address        Load       Tokens  Owns    Host ID                               Rack
> UN  188.87 GB  256     ?       dfc29863-c935-4909-9d7f-c59a47eda03d  rack1
> UN  198.37 GB  256     ?       12e7628b-7f05-48f6-b7e4-35a82010021a  rack1
> UN  191.88 GB  256     ?       26088392-f803-4d59-9073-c75f857fb332  rack1
> All nodes crashed due to power failure (know beforehand) and repair operations were started
for each node one at the time. Below is the behavior of SSTable count on different nodes.
New data was dumped simultaneously with repair operation.
> SEE FIGURE: sstable_count_figure1.png
> Vertical lines indicate following events.
> 1) Cluster was down due to power shutdown and was restarted. At the first vertical line
the repair operation (nodetool repair -pr) was started for the first node
> 2) Repair for the second repair operation was started after the first node was successfully
> 3) Repair for the third repair operation was started
> 4) Third repair operation was finished
> 5) One of the nodes crashed (unknown reason in OS level)
> 6) Repair operation (nodetool repair -pr) was started for the first node
> 7) Repair operation for the second node was started
> 8) Repair operation for the third node was started
> 9) Repair operations finished
> These repair operations are leading to huge amount of small SSTables covering the whole
time span of the data. The compaction horizon of DTCS was limited to 5 days (max_sstable_age_days)
due to the size of the SStables on the disc. Therefore, small SStables won't be compacted.
Below are the time graphs from SSTables after the second round of repairs.
> Status=Up/Down
> |/ State=Normal/Leaving/Joining/Moving
> --  Address        Load       Tokens  Owns    Host ID                               Rack
> UN  xx.xx.xx.170  663.61 GB  256     ?       dfc29863-c935-4909-9d7f-c59a47eda03d  rack1
> UN  xx.xx.xx.169  763.52 GB  256     ?       12e7628b-7f05-48f6-b7e4-35a82010021a  rack1
> UN  xx.xx.xx.168  651.59 GB  256     ?       26088392-f803-4d59-9073-c75f857fb332  rack1
> See files:
> node0_20150810_1017_time_graph.txt
> node1_20150810_1017_time_graph.txt
> node2_20150810_1017_time_graph.txt
> To get rid of the SStables the TimeWindowCompactionStrategy was taken into use. Window
size was set to 5 days. Cassandra version was updated to 2.1.8. Below figure shows the behavior
of SStable count. TWCS was taken into use 10.8.2015 at 13:10. The maximum amount of files
to be compacted in one task was limited to 32 files to avoid running out of disk space.
> See Figure: sstable_count_figure2.png
> Shape of the trend indicates clearly how selection of SStables for buckets based on size
affects. Combining files gets slower when files are getting bigger inside the time window.
When the time window does not have any more compactions to be done the next time window is
started. Combining small files is again fast and the number of SStables decreases quickly.
 Below are the time graphs for SStables when compactions were ready with TWCS. New data was
not dumped simultaneously with compactions.
> See files:
> node0_20150812_1531_time_graph.txt
> node1_20150812_1531_time_graph.txt
> node2_20150812_1531_time_graph.txt
> Datacenter: datacenter1
> =======================
> Status=Up/Down
> |/ State=Normal/Leaving/Joining/Moving
> --  Address        Load       Tokens  Owns    Host ID                               Rack
> UN  xx.xx.xx.170  436.17 GB  256     ?       dfc29863-c935-4909-9d7f-c59a47eda03d  rack1
> UN  xx.xx.xx.169  454.96 GB  256     ?       12e7628b-7f05-48f6-b7e4-35a82010021a  rack1
> UN  xx.xx.xx.168  439.13 GB  256     ?       26088392-f803-4d59-9073-c75f857fb332  rack1
> Data dumping was activated again and the SStable statistics were observed again on the
next morning.
> See files:
> node0_20150813_0835_time_graph.txt
> node1_20150813_0835_time_graph.txt
> node2_20150813_0835_time_graph.txt
> Since the data was dumped to the history the newest data did not come into the current
time window that is determined from the system time. Since new small SSTables (approximately
30- 50 MB in size) are appearing continuously the compaction ended up compacting together
one large SStable with several small files. The code was modified so that the current time
is determined from the newest time stamp in the SStables (like in DTCS).This modification
led to much more reasonable compaction behavior for the case when historical data is pushed
to the database. Below are the time grahps from nodes after one day. Size tiered compaction
was now able to work with newest files as intended while dumping data in real-time.
> See files:
> node0_20150814_1054_time_graph.txt
> node1_20150814_1054_time_graph.txt
> node2_20150814_1054_time_graph.txt
> The change in behavior is clearly visible in the compaction hierarchy graph below. TWCS
modification is visible starting from the line 39. See the description of the file format
> See file: 20150814_1027_compaction_hierarchy.txt
> The behavior of the TWCS looks really promising and works also in practice!!!
> We would like to propose some ideas for future development of the algorithm.
> 1) The current time window would be determined from the newest time stamp found in SSTables.
This allows the effective compaction of the SSTables when data is fed to the history in timely
order. In dumping process the time stamp of the column is set according to the time stamp
of the data sample.
> 2) The count of SSTables participating in one compaction could be limited either by the
number of files given by max_threshold OR by the sum of size of files selected for the compaction
bucket. File size limitation would prevent combining a large files together potentially causing
out of disk space situation or extremely long lasting compaction tasks.
> 3) Now time windows are handled one by one starting from the newest. This will not lead
to the fastest decrease in SStable count. An alternative might a round-robin approach in which
time windows are stepped through and only one compaction task for that given time window is
done and then moving to the next time window.
> Side note: while observing the compaction process it appears that compaction is intermittently
using two threads for the compaction. However, sometimes during a long lasting compaction
task (hours) another thread was not kicking in and working with small SSTables even if there
were thousands of those available for compactions.

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