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From Apache Wiki <wikidi...@apache.org>
Subject [Cassandra Wiki] Trivial Update of "Durability" by JonathanEllis
Date Wed, 07 Apr 2010 16:32:40 GMT
Dear Wiki user,

You have subscribed to a wiki page or wiki category on "Cassandra Wiki" for change notification.

The "Durability" page has been changed by JonathanEllis.
http://wiki.apache.org/cassandra/Durability?action=diff&rev1=1&rev2=2

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

  
  The naive way to provide durability is to write and [[http://linux.die.net/man/2/fsync|fsync]]
your data files with each write, but this is prohibitively slow in practice because the disk
needs to do random seeks to write the data to the write location on the physical platters.
 (Remember that each seek costs 5-10ms on rotational media.)
  
- Like most modern systems, Cassandra provides durability by appending writes to a commitlog
first.  This means that only the commitlog needs to be fsync'd, which, if the commitlog is
on its own volume, obviates the need for seeking since the commitlog is append-only.  Implementation
details are in ArchitectureCommitlog.
+ Like most modern systems, Cassandra provides durability by appending writes to a commitlog
first.  This means that only the commitlog needs to be fsync'd, which, if the commitlog is
on its own volume, obviates the need for seeking since the commitlog is append-only.  Implementation
details are in ArchitectureCommitLog.
  
  Commitlog behavior is configurable with the CommitLogSync, CommitLogSyncPeriodInMS, and
CommitLogSyncBatchWindowInMS.  See the sample storage configuration for details on using these.
 (CommitLogSyncPeriodInMS and CommitLogSyncBatchWindowInMS correspond to the [[http://www.postgresql.org/docs/8.4/static/runtime-config-wal.html|PostgreSQL
settings]] of wal_writer_delay and commit_delay, respectively; however, unlike PostgreSQL,
Cassandra does not allow turning fsync off completely.)
  

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