</section>
<section remap="h3">
<title><indexterm><primary>storage</primary><secondary>configuring</secondary><tertiary>external journal</tertiary></indexterm>Choosing Parameters for an External Journal</title>
- <para>If you have configured a RAID array and use it directly as an OST, it contains both data and metadata. For better performance, we recommend putting the OST journal on a separate device, by creating a small RAID 1 array and using it as an external journal for the OST.</para>
- <para>In a Lustre file system, the default journal size is 400 MB. A journal size of up to 1
- GB has shown increased performance but diminishing returns are seen for larger journals.
- Additionally, a copy of the journal is kept in RAM. Therefore, make sure you have enough
- memory available to hold copies of all the journals.</para>
+ <para>If you have configured a RAID array and use it directly as an OST,
+ it contains both data and metadata. For better performance, we
+ recommend putting the OST journal on a separate device, by creating a
+ small RAID 1 array and using it as an external journal for the OST.
+ </para>
+ <para>In a typical Lustre file system, the default OST journal size is
+ up to 1GB, and the default MDT journal size is up to 4GB, in order to
+ handle a high transaction rate without blocking on journal flushes.
+ Additionally, a copy of the journal is kept in RAM. Therefore, make
+ sure you have enough RAM on the servers to hold copies of all journals.
+ </para>
<para>The file system journal options are specified to <literal>mkfs.lustre</literal> using
the <literal>--mkfsoptions</literal> parameter. For example:</para>
<screen>--mkfsoptions "<replaceable>other_options</replaceable> -j -J device=/dev/mdJ" </screen>
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