Shared SAN Cluster Configurations Get A Firm Foundation - VERITAS SANPoint Foundation Suite HA - Product Information

Computer Technology Review, March, 2001 by Christine Chudnow

A SAN is a storage networking architecture that allows for more efficient use of storage capacity by offloading storage from the LAN to a dedicated storage network. However, one of the features SAN administrators most want has only recently been widely available: the ability to virtualize data across a heterogeneous SAN. What appeared to users as mere network data appeared to SAN administrators as a nightmare of complexity, disk additions, and bad games of "guess the server" as they tossed data around from storage device to storage device.

A new product family that adds virtualization and other benefits to clustered server environments is VERITAS SANPoint Foundation Suite HA. The suite extends VERITAS File System and Volume Manager to support concurrent data sharing among clustered servers in a SAN. It also incorporates VERITAS Cluster Server's file system capabilities and internode communications across the servers. This impacts a number of applications, including highly available configurations such as databases, Web farms, workflow applications with large files, and offhost backups running on separate servers. VERITAS claims four primary features:

Concurrent access to shared tiles. Multiple servers mount and access the same file system on shared media, with no required modifications to existing applications.

File system integrity in a shared environment. Controls access to the file system structure using a global lock manager, and manages cache coherence and locking. Systems accessing shared file systems always see the same information.

Fast failover for high availability environments. Provides robust application-level failover from VERITAS Cluster Server.

Clusterwide management of SAN data. Allows clusterwide logical device naming and volume and file system operations.

Clusters

The suite works at the server level in clustering groups. Clustering consists of servers connected to the same storage devices, accessible by the same clients, and coordinated by a cluster server application. They offer a number of distinct advantages to IT administrators, including failover in the case of server failure, the ability to repartition workload on multiple servers, alternate network links in case of link failure, disaster recovery, and minimizing the management of multiple individual systems.

The Foundation Suite incorporates VERITAS Volume Manager, File System, and interconnection technologies from Cluster Server. It also adds unique technologies from Cluster Volume Manager and Cluster File System.

Cluster Volume Manager (CVM) makes use of the foundational technologies of Volume Manager (VxVM). VxVM aggregates disks or hardware-based RAID arrays into flexible logical volumes. Operating on a three-tier storage object architecture, VxVM aggregates block ranges on physical disks into plexes, which represent complete and consistent copies of the volume content. Each plex offers failure tolerance and data mapping properties. It then aggregates the plexes into volumes that appear as disks to file systems, database managers, and applications. CVM, which is a separate licensed product, acts in conjunction with VxVM to add virtualization features as well as application and volume failover. It can manage both physical disks and the virtual disks exported by hardware RAID array subsystems.

Just as CVM is based on VxVM, CFS is based on VERITAS File System (VxFS). The Cluster File System (CFS) enables several clustered servers to mount and use a file system as if all applications using the file system were running on the same server. It uses a master-client model to manage file system metadata on shared volumes, with the first server to mount each CFS file system becoming its master while all other cluster nodes become clients. The CFS master node makes all metadata updates and maintains the metadata update intent log, while applications access the user data in files directly from the server on which they are running.

CVM offers features found in VxFS including managing space by concisely mapping files up to a terabyte in size, fast recovery from most system crashes by tracking recent file system metadata updates, online capability to extend and defragment active file systems, and quick I/O features that bypass database kernel locking by treating files as raw partitions. This last feature also enables 32 bit applications to avail themselves of a system cache larger than 4GB.

CFS extends these capabilities to clusters and adds the following:

* Freezes the file system state throughout the cluster. This allows administrators to perform certain operations on applications that require a consistent on-disk image of a file system.

* Allows both clusterwide and local file system mounting, allowing administrators to choose to share data among cluster nodes.

* Enables a node by node upgrade of CFS itself, allowing the cluster as a whole to operate throughout the upgrade process.

Some examples of Foundation Suite applications include continuous availability environments, parallel applications, workflow files, and backup operations.