Virtual disks

Whether a vdisk of a particular capacity can be created in a declustered array depends on its redundancy code, the number of pdisks and equivalent spare capacity in the array, and other small IBM Storage Scale RAID overhead factors. The mmcrvdisk command can automatically configure a vdisk of the largest possible size given 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 3-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 later.