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From Jeff Jirsa <jeff.ji...@crowdstrike.com>
Subject Re: DTCS SSTable count issue
Date Mon, 11 Jul 2016 20:38:18 GMT
DTCS is deprecated in favor of TWCS in new versions, yes. 

 

Worth mentioning that you can NOT disable blocking read repair which comes naturally if you
use CL > ONE. 

 

>  Also instead of major compactions (which comes with its set of issues / tradeoffs too)
you can think of a script smartly using sstablemetadata to find the sstables holding too much
tombstones and running single SSTable compactions on them through JMX and user defined compactions.
Meanwhile if you want to do it manually, you could do it with something like this to know
the tombstone ratio from the biggest sstable:

 

The tombstone compaction options basically do this for you for the right settings (unchecked
tombstone compaction = true, set threshold to 85% or so, don’t try to get clever and set
it to something very close to 99%, the estimated tombstone ratio isn’t that accurate)

 

-          Jeff

 

 

From: Alain RODRIGUEZ <arodrime@gmail.com>
Reply-To: "user@cassandra.apache.org" <user@cassandra.apache.org>
Date: Monday, July 11, 2016 at 1:05 PM
To: "user@cassandra.apache.org" <user@cassandra.apache.org>
Subject: Re: DTCS SSTable count issue

 

@Jeff 

 

Rather than being an alternative, isn't your compaction strategy going to deprecate (and finally
replace) DTCS ? That was my understanding from the ticket CASSANDRA-9666.

@Riccardo 

 

If you are interested in TWCS from Jeff, I believe it has been introduced in 3.0.8 actually,
not 3.0.7 https://github.com/apache/cassandra/blob/cassandra-3.0/CHANGES.txt#L28. Anyway,
you can use it in any recent version as compactions strategies are pluggable. 

 

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.

 

I observed the same issue recently and I am confident that TWCS will solve this tombstone
issue, but it is not tested on my side so far. Meanwhile, be sure you have disabled any "read
repair" on tables using DTCS and maybe hints as well. It is a hard decision to take as you'll
loose 2 out of 3 anti entropy systems, but DTCS behaves badly with those options turned on
(TWCS is fine with it). The last anti-entropy being a full repair that you might already not
be running as you only do inserts...

 

Also instead of major compactions (which comes with its set of issues / tradeoffs too) you
can think of a script smartly using sstablemetadata to find the sstables holding too much
tombstones and running single SSTable compactions on them through JMX and user defined compactions.
Meanwhile if you want to do it manually, you could do it with something like this to know
the tombstone ratio from the biggest sstable:

du -sh /path_to_a_table/* | sort -h | tail -20 | awk "{print $1}" && du -sh /path_to_a_table/*
| sort -h | tail -20 | awk "{print $2}" | xargs sstablemetadata | grep tombstones

And something like this to run a user defined compaction on the ones you chose (big sstable
with high tombstone ratio):

echo "run -b org.apache.cassandra.db:type=CompactionManager forceUserDefinedCompaction <Data_db_file_name_without_path>"
| java -jar jmxterm-version.jar -l <ip>:<jmx_port>

note: you have to download jmxterm (or use any other jmx tool).

 

Did you give a try to the unchecked_tombstone_compaction as well (compaction options at the
table level)? Feel free to set this one to true. I think it could be the default. It is safe
as long as your machines have some more resources available (not that much). That's the first
thing I would do.

 

Also if you use TTL only, feel free to reduce the gc_grace_seconds, this will probably help
having tombstones removed. I would start with other solutions first. Keep in mind that if
someday you perform deletes, this setting could produce you some Zombies (data coming back),
if you don't run repair in the gc_grace_seconds for the entire ring.

C*heers,

-----------------------

Alain Rodriguez - alain@thelastpickle.com

France

 

The Last Pickle - Apache Cassandra Consulting

http://www.thelastpickle.com

 

2016-07-07 19:25 GMT+02:00 Jeff Jirsa <jeff.jirsa@crowdstrike.com>:

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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