Overview of fixed-sized provisioning

Two traditional fixed-size host-based volume management methods typically are used on open systems to organize storage space on a server. One method is the direct use of physical volumes as devices for use either as raw space or as a local file system. These are fixed-size volumes with a fixed number of disks, and as such, each has a certain inherent physical random input/output operation per second (IOPS) or sequential throughput (megabytes per second) capacity. A system administrator manages the aggregate server workloads against them. As workloads exceed the volume's available space or its IOPS capacity, the data is manually moved onto a larger or faster volume, if possible.

The following figure illustrates a simple fixed-size provisioning environment using individual fixed volumes on a host:

The other method is to use a host-based Logical Volume Manager (LVM) where the planned workloads require either more space or IOPS capacity than the individual physical volumes can provide. LVM is the disk management feature available on UNIX-based operating systems, including Linux, that manages their logical volumes.

The following illustrates a fixed-size provisioning environment using LUNs in host-managed logical volumes:

With either method, hosts recognize the size as fixed regardless of the actual used size. Therefore, it is not necessary to expand the volume (LDEV) size in the future if the actual used size does not exceed the fixed size.

When such a logical volume runs out of space or IOPS capacity, you can replace it with one that was created with even more physical volumes then copy over all of the user data. In some cases, it is best to add a second logical volume then manually relocate only part of the existing data to redistribute the workload across two such volumes. These two logical volumes would be mapped to the server using separate host paths.

Disadvantages

Some disadvantages to using fixed-sized provisioning are:

  • If you use only part of the entire capacity specified by an emulation type, the rest of the capacity is wasted.
  • After creating fixed-sized volumes, typically some physical capacity will be wasted.
  • In a fixed-sized environment, manual intervention can become a costly and tedious exercise when a larger volume size is required.

When to use fixed-sized provisioning

Use fixed-sized provisioning when custom-sized provisioning is not supported.

Parent Topic
  • Basic provisioning

    Several basic provisioning techniques traditionally are used to manage storage volumes. These strategies are useful in specific scenarios based on user needs, such as what type of storage to use or how to manually size volumes. Basic provisioning relies on carving up physical storage into logical devices. Custom-size volumes are configured by using the Virtual LUN software.