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1.2.3.2. Storage

An environment that uses distributed computing uses multiple instances of shared storage. This can mean one of two things:
  • Multiple systems storing data in a single location
  • A storage unit (e.g. a volume) composed of multiple storage appliances
The most familiar example of storage is the local disk drive mounted on a system. This is appropriate for IT operations where all applications are hosted on one host, or even a small number of hosts. However, as the infrastructure scales to dozens or even hundreds of systems, managing as many local storage disks becomes difficult and complicated.
Distributed storage adds a layer to ease and automate storage hardware administration as the business scales. Having multiple systems share a handful of storage instances reduces the number of devices the administrator needs to manage.
Consolidating the storage capabilities of multiple storage appliances into one volume helps both users and administrators. This type of distributed storage provides a layer of abstraction to storage pools: users see a single unit of storage, which an administrator can easily grow by adding more hardware. Some technologies that enable distributed storage also provide added benefits, such as failover and multipathing.
NFS

Network File System (NFS) allows multiple servers or users to mount and use the same instance of remote storage via TCP or UDP. NFS is commonly used to hold data shared by multiple applications. It is also convenient for bulk storage of large amounts of data.

SAN

Storage Area Networks (SANs) use either Fibre Channel or iSCSI protocol to provide remote access to storage. Fibre Channel infrastructure (such as Fibre Channel host bus adapters, switches, and storage arrays) combines high performance, high bandwidth, and massive storage. SANs separate storage from processing, providing considerable flexibility in system design.

The other major advantage of SANs is that they provide a management environment for performing major storage hardware administrative tasks. These tasks include:
  • Controlling access to storage
  • Managing large amounts of data
  • Provisioning systems
  • Backing up and replicating data
  • Taking snapshots
  • Supporting system failover
  • Ensuring data integrity
  • Migrating data
GFS2

The Red Hat Global File System 2 (GFS2) file system provides several specialized capabilities. The basic function of GFS2 is to provide a single file system, including concurrent read/write access, shared across multiple members of a cluster. This means that each member of the cluster sees exactly the same data "on disk" in the GFS2 filesystem.

GFS2 allows all systems to have concurrent access to the "disk". To maintain data integrity, GFS2 uses a Distributed Lock Manager (DLM), which only allows one system to write to a specific location at a time.
GFS2 is especially well-suited for failover applications that require high availability in storage.
For further information about GFS2, refer to the Global File System 2. For further information about storage in general, refer to the Storage Administration Guide. Both are available from http://access.redhat.com/site/documentation/Red_Hat_Enterprise_Linux/.