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Figure 1-61 Example of declustered arrays and recovery groups in storage JBOD
Virtual and physical disks
A VDisk is a type of NSD that is implemented by GPFS Native RAID across all the pdisks of a
declustered array. Multiple VDisks are defined within a declustered array, typically
Reed-Solomon VDisks for GPFS user data and replicated VDisks for GPFS metadata.
Virtual disks
Whether a VDisk of a particular capacity is created in a declustered array depends on its
redundancy code, the number of pdisks and equivalent spare capacity in the array, and other
small GPFS Native RAID overhead factors. The mmcrvdisk command automatically
configures a VDisk of the largest possible size a redundancy code and configured spare
space of the declustered array.
In general, the number of pdisks in a declustered array cannot be less than the widest
redundancy code of a VDisk plus the equivalent spare disk capacity of a declustered array.
For example, a VDisk that uses the 11-strip-wide 8 + 3p Reed-Solomon code requires at least
13 pdisks in a declustered array with the equivalent spare space capacity of two disks. A
VDisk that uses the three-way replication code requires at least five pdisks in a declustered
array with the equivalent spare capacity of two disks.
VDisks are partitioned into virtual tracks, which are the functional equivalent of a GPFS block.
All VDisk attributes are fixed at creation and cannot be altered.
Physical disks
A pdisk is used by GPFS Native RAID to store user data and GPFS Native RAID internal
configuration data.
A pdisk is either a conventional rotating magnetic-media disk (HDD) or a solid-state disk
(SSD). All pdisks in a declustered array must have the same capacity.
Pdisks are also assumed to be dual-ported with one or more paths that are connected to the
primary GPFS Native RAID server and one or more paths that are connected to the backup
server. Often there are two redundant paths between a GPFS Native RAID server and
connected JBOD pdisks.
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Chapter 1. Understanding the IBM Power Systems 775 Cluster
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