Storage pools

Storage Subsystem Administration Guide for Hitachi NAS Platform

Part Number

A NAS server storage pool (known as a "span" in the command line interface) is the logical container for a collection of four or more system drives (SDs). There are two types of NAS server storage pools:

  • An untiered storage pool is made up of system drives (SDs) created on one or more storage systems within the same tier of storage (storage systems with comparable performance characteristics). To create an untiered storage pool, there must be at least four available and unused system drives on the storage system from which the SDs in the storage pool are taken.
  • A tiered storage pool is made up of system drives (SDs) created on storage systems with different performance characteristics. Typically, a tiered storage pool is made up of SDs from high-performance storage such as flash memory, and SDs from lower-performance storage such as SAS (preferably) or NL SAS (near line SAS). You can, however, create a tiered storage pool from SDs on storage systems using any storage technology, and you can create both tiers on the same storage system.

NAS server storage pools:

  • Can be expanded as additional SDs are created in the storage system, and a storage pool can grow to a maximum of 1 PiB or 256 SDs. Expanding a NAS server storage pool does not interrupt network client access to storage resources. SDs may be based on parity groups, or on HDP DP-Vols (preferably).
  • Support two types of thin provisioning:
    • NAS server storage pools can be thinly provisioned when created using SDs based on HDP DP-Vols.
    • File system thin provisioning, through the use of the NAS server filesystem-thin command and file system confinement and auto-expansion. This type of thin provisioning allows you to create a small file system, which can then automatically expand when necessary, and that ability saves the overhead associated with sustaining unnecessary storage.

      When file system thin provisioning is enabled, the server reports to protocol clients (though not at the CLI or in the GUI) that the file system is larger than it really is: either the capacity to which you have confined it or the maximum capacity to which it can ever grow, whichever is smaller.

      Refer to the Command Line Reference for more information on the filesystem-thin command.

  • Contain a single stripeset on initial creation. Each time the storage pool is expanded, another stripeset is added, up to a maximum of 64 stripesets (after creation, a storage pool can be expanded a maximum of 63 times). As HDP is the preferred method to provision DP Vols to the NAS, Hitachi recommends thin provisioning the pool 200 - 300 percent, which lessens the likelihood of stripset expansions.
  • Contain the file systems and enable the user to manage the file system settings that are common to all file systems in the storage pool. For example, the settings applied to a storage pool can either allow or constrain the expansion of all file systems in the storage pool.
    Note: By default, there is a limit of 32 file systems per storage pool. Recently deleted file systems that are still in the recycle bin do not count towards this number. It is possible to increase this limit using the filesystem-create CLI command with the --exceed-safe-count option. See the command man page for details.