CODEWITHSUNDEEP
cassandra
Kurt
Kurt

Reputation: 161

New cassandra node should finish compaction before joining ring

I’d like to know if there is a way to have a Cassandra node join the ring only after it has finished streaming and compaction. The issue I’m experiencing is that when I add a node to my cluster, it streams data from the other nodes then joins the ring, at this point it begins a lot of compactions, and the compactions take a very long time to complete (greater than a day), during this time CPU utilization on that node is nearly 100%, and bloom filter false positive ratio is very high as well which happens to be relevant to my use case. This causes the whole cluster to experience an increase in read latency, with the newly joined node in particular having 10x the typical latency for reads.

I read this post http://www.datastax.com/dev/blog/bootstrapping-performance-improvements-for-leveled-compaction which has this snippet about one way to possibly improve read latency when adding a node.

“Operators usually avoid this issue by passing -Dcassandra.join_ring=false when starting the new node and wait for the bootstrap to finish along with the followup compactions before putting the node into the ring manually with nodetool join”

The documentation on the join_ring option is pretty limited but after experimenting with it it seems that streaming data and the later compaction can’t be initiated until after I run nodetool join for the new host, so I’d like to know how or if this can be achieved.

Right now my use case is just deduping records being processed by a kafka consumer application. The table in cassandra is very simple, just a primary key, and the queries are just inserting new keys with a ttl of several days and checking existence of a key. The cluster needs to perform 50k reads and 50k writes per second at peak traffic.

I’m running cassandra 3.7 My cluster is in EC2 originally on 18 m3.2xlarge hosts. Those hosts were running at very high (~90%) CPU utilization during compactions which was the impetus for trying to add new nodes to the cluster, I’ve since switched to c3.4xlarge to give more CPU without having to actually add hosts, but it’d be helpful to know at what CPU threshold I should be adding new hosts since waiting until 90% is clearly not safe, and adding new hosts exacerbates the CPU issue on the newly added host.

CREATE TABLE dedupe_hashes (
   app int,
   hash_value blob,
   PRIMARY KEY ((app, hash_value))
) WITH bloom_filter_fp_chance = 0.01
       AND caching = {'keys': 'ALL', 'rows_per_partition': 'NONE'}
    AND comment = ''
    AND compaction = {'class': 'org.apache.cassandra.db.compaction.SizeTieredCompactionStrategy', '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 = 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 = '90PERCENTILE';

Upvotes: 2

Views: 1011

Answers (1)

J KVS
J KVS

Reputation: 101

One thing to check that may help avoid the 100% CPU due to compactions is the setting for "compaction_throughput_mb_per_sec". By default it should be 16MB but check if you have this disabled (set to 0).

nodetool getcompactionthroughput

You can also set the value dynamically

nodetool setcompactionthroughput 16

(set in cassandra.yaml to persist after restart).

Upvotes: 0

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