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From Mohica Jasha <mohica.ja...@gmail.com>
Subject Cassandra hopefully durable writes (commitlog_sync)
Date Wed, 23 Apr 2014 15:08:30 GMT
There is a discrepancy between the two documentation:

http://wiki.apache.org/cassandra/Durability

Cassandra's default configuration sets the commitlog_sync mode to periodic,
> causing the commitlog to be synced every commitlog_sync_period_in_ms milliseconds,
> so you can potentially lose up to that much data if all replicas crash
> within that window of time.
>


http://www.datastax.com/documentation/cassandra/2.0/cassandra/dml/dml_durability_c.html

Writes in Cassandra are durable. All writes to a replica node are recorded
> both in memory and in a commit log on disk before they are acknowledged as
> a success. If a crash or server failure occurs before the memory tables are
> flushed to disk, the commit log is replayed on restart to recover any lost
> writes.
>

I wonder which is correct?
Does Cassandra (default configuration) wait till it persists the update to
its commit log before it acks back the write? or doesn't it?

I wonder what happens if due to power outage all replicas die at the same
time? Can such a power outage cause writes kept in memory within that 10
second window to be lost?


Mohica

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