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From Denis Gabaydulin <gaba...@gmail.com>
Subject [problem with OOM in nodes]
Date Thu, 20 Sep 2012 11:27:05 GMT
Hi, all!

We have a cluster with virtual 7 nodes (disk storage is connected to
nodes with iSCSI). The storage schema is:

Reports:{
    1:{
        1:{"value1":"some val", "value2":"some val"},
        2:{"value1":"some val", "value2":"some val"}
        ...
    },
    2:{
        1:{"value1":"some val", "value2":"some val"},
        2:{"value1":"some val", "value2":"some val"}
        ...
    }
    ...
}

create keyspace osmp_reports
  with placement_strategy = 'SimpleStrategy'
  and strategy_options = {replication_factor : 4}
  and durable_writes = true;

use osmp_reports;

create column family QueryReportResult
  with column_type = 'Super'
  and comparator = 'BytesType'
  and subcomparator = 'BytesType'
  and default_validation_class = 'BytesType'
  and key_validation_class = 'BytesType'
  and read_repair_chance = 1.0
  and dclocal_read_repair_chance = 0.0
  and gc_grace = 432000
  and min_compaction_threshold = 4
  and max_compaction_threshold = 32
  and replicate_on_write = true
  and compaction_strategy =
'org.apache.cassandra.db.compaction.SizeTieredCompactionStrategy'
  and caching = 'KEYS_ONLY';

=============================================

Read/Write CL: 2

Most of the reports are small, but some of them could have a half
mullion of rows (xml). Typical operations on this dataset is:

count report rows by report_id (top level id of super column);
get columns (report_rows) by range predicate and limit for given report_id.

A data is written once and hasn't never been updated.

So, time to time a couple of nodes crashes with OOM exception. Heap
dump says, that we have a lot of super columns in memory.
For example, I see one of the reports is in memory entirely. How it
could be possible? If we don't load the whole report, cassandra could
whether do this for some internal reasons?

What should we do to avoid OOMs?

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