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Shared SAN Cluster Configurations Get A Firm Foundation.

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.


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.

Continuous availability. Failover abilities are a basic feature of cluster management. In this model, servers in a cluster configuration serve separate file systems. If one server fails, the other identifies the failure, mounts the failed server's file system, and restarts the client application. But like other types of non-virtualized storage models, IT administrators must still assign and reassign storage space on individual servers, hopefully without taking down a critical application or system. Storage virtualization simplifies administration, improves service and critical data availability, and improves performance due to I/O load balancing if the volumes holding the file systems are striped across more disks.

Parallel application: Web server. Server farms are a popular configuration of high-transaction environments such as Internet server clusters. Most of them feature a load balancing facility in addition to server failover and the ability to add servers and data copies at will. This model is demanding of administrator time and resources since multiple copies of data on servers make clusterwide updates quite challenging. This results in a high cost of incremental storage and administration, and undermines the integrity of data by maintaining multiple copies. Shared data clusters enable all Web servers to work from one data image, no matter which server handles a request. Adding capacities no longer requires restructuring the operation.

Workflow application: Video production. In workflow applications, a single piece of work flows (or lurches) from server to server. Among these types of applications, video post-production, and its huge files, is possibly the most demanding of space and resources. Clusters and shared storage eliminate the need to transfer these files via tape or over the network, but the problem still remains of locking out one user's access to the data while a previous workstation is still manipulating it. A cluster can ideally provide universal interconnection of computers and data, so video objects are passed between stations only when not in use by another workstation.

Backup Application: Block level incremental database backups. Vendors have flooded the backup market with devices and applications, seeking to relieve IT administrators of horrendous backup realities. Increasing application demands, larger databases, and complex systems make it difficult to back up and restore in a reasonable period of time without denying service to network users. VERITAS has identified two fundamental backup problems: 1) Obtaining consistent point-in-time backups of large file systems or databases without blocking application access, and 2) backing up large file systems or databases without disrupting operational client traffic or server I/O. When used with VERITAS NetBackup, the Foundation Suite allows off-host backups from different servers in the same cluster, accessing the same shared data. This allows administrators to create point-in-time snapshots of critical data such as Oracle databases, then to use a separate server running NetBackup to back up as per the snapshot.

Shared data cluster configurations offer a number of advantages over single host or shared-nothing clusters. In addition to providing this technology, the Foundation Suite leverages the File System and Volume Manager products and offers cluster virtualization, fast failover, virtual single file systems, and global locks to manage access to data and provide cache coherency.
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Title Annotation:Product Information; VERITAS SANPoint Foundation Suite HA
Author:Chudnow, Christine
Publication:Computer Technology Review
Date:Mar 1, 2001
Previous Article:Storage Virtualization--Strategic For SANs.
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