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From Jeff Jirsa <jeff.ji...@crowdstrike.com>
Subject Re: DTCS SSTable count issue
Date Thu, 07 Jul 2016 17:25:52 GMT
48 sstables isn’t unreasonable in a DTCS table. It will continue to grow over time, but ideally
data will expire as it nears your 90 day TTL and those tables should start dropping away as
they age.

 

3.0.7 introduces an alternative to DTCS you may find easier to use called TWCS. It will almost
certainly help address the growing sstable count.  

 

 

 

From: Riccardo Ferrari <ferrarir@gmail.com>
Reply-To: "user@cassandra.apache.org" <user@cassandra.apache.org>
Date: Thursday, July 7, 2016 at 6:49 AM
To: "user@cassandra.apache.org" <user@cassandra.apache.org>
Subject: DTCS SSTable count issue

 

Hi everyone, 

 

This is my first question, apologize may I do something wrong.

 

I have a small Cassandra cluster build upon 3 nodes. Originally born as 2.0.X cluster was
upgraded to 2.0.15 then 2.1.13 and finally to 3.0.4 recently 3.0.6. Ubuntu is the OS.

 

There are few tables that have DateTieredCompactionStrategy and are suffering of constantly
growing SSTable count. I have the feeling this has something to do with the upgrade however
I need some hint on how to debug this issue.

 

Tables are created like:

CREATE TABLE <table> (

 ...

PRIMARY KEY (...)

) WITH CLUSTERING ORDER BY (...)

    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.DateTieredCompactionStrategy',
'max_threshold': '32', 'min_threshold': '4'}

    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 = 0.1

    AND default_time_to_live = 7776000

    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';

 

and this is the "nodetool cfstats" output for that table:

Read Count: 39

Read Latency: 85.03307692307692 ms.

Write Count: 9845275

Write Latency: 0.09604882382665797 ms.

Pending Flushes: 0

Table: <table>

SSTable count: 48

Space used (live): 19566109394

Space used (total): 19566109394

Space used by snapshots (total): 109796505570

Off heap memory used (total): 11317941

SSTable Compression Ratio: 0.22632301701483284

Number of keys (estimate): 2557

Memtable cell count: 0

Memtable data size: 0

Memtable off heap memory used: 0

Memtable switch count: 828

Local read count: 39

Local read latency: 93.051 ms

Local write count: 9845275

Local write latency: 0.106 ms

Pending flushes: 0

Bloom filter false positives: 2

Bloom filter false ratio: 0.00000

Bloom filter space used: 10200

Bloom filter off heap memory used: 9816

Index summary off heap memory used: 4677

Compression metadata off heap memory used: 11303448

Compacted partition minimum bytes: 150

Compacted partition maximum bytes: 4139110981

Compacted partition mean bytes: 13463937

Average live cells per slice (last five minutes): 59.69230769230769

Maximum live cells per slice (last five minutes): 149

Average tombstones per slice (last five minutes): 8.564102564102564

Maximum tombstones per slice (last five minutes): 42

 

According to the "nodetool compactionhistory <keyspace>.<table>"

the oldest timestamp is "Thu, 30 Jun 2016 13:14:23 GMT"

and the most recent one is "Thu, 07 Jul 2016 12:15:50 GMT" (THAT IS TODAY)

 

However the table count is still very high compared to tables that have a different compaction
strategy. If I run a "nodetool compact <table>" the SSTable count decrease dramatically
to a reasonable number.

I read many articles including: http://www.datastax.com/dev/blog/datetieredcompactionstrategy
however I can not really tell if this is an expected behavior.

What concerns me is that I have an high tombstone read count despite those are insert only
tables. Compacting the table make the tombstone issue disappear. Yes, we are using TTL to
expire data after 3 months and I have not touch the GC grace period.

Looking at the file system I see the very first *-Data.db file that is 15GB then there are
all the other 43 *-Data.db files that are ranging from 50 to 150MB in size.

 

How can I debug this mis-compaction issue? Any help is much appreciated

Best,


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