Chapter 4. Management and maintenance 213
germany:/gpfs#mmlsnsd
File system Disk name NSD servers
---------------------------------------------------------------------------
(free disk) nsdhdisk2 usa,germany
(free disk) nsdhdisk3 usa,germany
(free disk) nsdhdisk4 usa,germany
(free disk) nsdhdisk5 usa,germany
4.2.6 Enabling GPFS replication
GPFS provides multiple features to increase data availability of your system. One feature is
the file system synchronous replication. You can set the GPFS replication to work on all the
file systems or only on some files. Currently the maximum number of copies allowed by GPFS
is two. This means that two copies of data and metadata are kept replicated to ensure data
availability. You can choose to have only metadata replicated to save disk space and to
increase performance because enabled replicas requires additional write times. GPFS replica
is based on failure group configuration. When you add new disks to the GPFS file system, you
can configure to which failure group the disks belong. You have to configure the same failure
group for disks that have in common the same point of failure, for example, the same disk
storage system. If you have one storage system in one location and one storage system in
another location, you configure the disks belonging to separate storage systems in separate
failure groups. In this way, GPFS knows that it has to replicate data and metadata from one
storage system to the other. In case of a complete storage system failure, GPFS has all the
data correctly replicated on the other storage environment.
This section describes the following replica characteristics:
Default metadata
Maximum metadata
Default data
Maximum data
Additional information about the GPFS replication parameters is in the General Parallel File
System Advanced Administration Guide Version 3 Release 4, SC23-5182.
Default metadata replicas
The default number of copies of metadata for all files in the file system may be specified at file
system creation by using the -m option on the mmcrfs command, or changed at a later time by
using the -m option on the mmchfs command. This value must be equal to or less than
MaxMetadataReplicas, and cannot exceed the number of failure groups with disks that can
store metadata. The allowable values are 1 or 2, with a default of 1.
Maximum metadata replicas
The maximum number of copies of metadata for all files in the file system can be specified at
file system creation by using the -M option on the mmcrfs command. The default is 2. The
allowable values are 1 or 2, but it cannot be lower than the value of DefaultMetadataReplicas.
This value cannot be changed.
Default data replicas
The default replication factor for data blocks may be specified at file system creation by using
the -r option on the mmcrfs command, or changed at a later time by using the -r option on the
mmchfs command. This value must be equal to or less than MaxDataReplicas, and the value

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