Easing the Migration from Parallel to Serial Storage

Rapid data growth at companies of all sizes is sending IT departments scrambling for cost-effective ways to store more information under tightening budgets.

New storage products are emerging with serial architectures that deliver higher performance, capacity scalability and denser designs, while simultaneously reducing total storage ownership costs. One of the biggest hurdles, however, to the adoption of storage systems based on serial technologies is the difficult process of moving mission-critical data from parallel technology to new serial-architecture storage with little or no downtime. Switching to new systems in enterprise environments can be especially challenging because of the need to schedule downtime for the many application servers that must run around the clock.

Challenges of migrating data

Migrating data to serial-based systems can be complex and labor-intensive because of the many application servers, operating systems, file systems, storage devices and networks used in today's computing environments. The most formidable challenge IT departments face in migrating data is protecting it against loss and corruption and minimizing disruptions to ongoing business operations.

Since parallel-technology disk drives are incompatible with serial-technology interfaces, data can't be migrated by simply sharing storage across common interface adapters and copying the data. Instead, the new serial storage subsystem must be attached on a separate adapter within the server or on a separate storage server altogether.

When it's not possible to establish a direct connection between existing and new storage, data can be migrated using network infrastructures such as a LAN, WAN or SAN in conjunction with mirroring technologies designed to ensure continuous data availability.

There are two traditional approaches to migrating data: staging data from the storage device on either tape or another storage medium and restore it to the new device, or using host-based software. Offloading data to a temporary storage device involves taking a point-in-time copy of data and staging it for copying to the temporary device, installing the new storage and copying data from the temporary device to the new one. The process often is simple but can require significant user downtime. To maintain data integrity, user access to stored data generally must be interrupted throughout backup and restoration, a process that can't be used in environments requiring continuous data access.

A less intrusive alternative is to migrate data using host-based software that provides users with limited access as data is being copied. The host software captures any I/O operation performed on the old storage system and replicates it on the new storage devices.

Using the network for data migration

To migrate and expand storage in ways that are transparent to users and to ensure continuous data access, organizations can leverage three well-entrenched storage infrastructures -- direct-attached storage (DAS), network-attached storage (NAS), or storage-area networks (SAN) -- and emerging storage management tools such as storage virtualization, DAS spillover and DAS evacuation.

Storage virtualization involves combining direct- and fabric-attached storage into a logical, or virtual, pool that the user sees as direct-attached storage to simplify allocation and management. The technology separates logical storage from physical storage by adding a vendor- and platform-neutral management layer between the servers and storage devices. Storage virtualization gives organizations an easy way to add new storage technology such as serial-attached SCSI to an existing storage infrastructure, logically display the two distinct subsystems as one, and migrate data to the new storage capacity.

With DAS spillover, a DAS volume is extended to fabric-attached storage to create a virtual storage pool that, like virtualization, appears as direct-attached storage to the user. DAS spillover can be performed independent of the application, operating system and DAS vendor. It gives fabric-attached storage the same look and ease of manageability as DAS storage by using existing management tools. Chief among its benefits, DAS spillover doesn't disrupt the infrastructure, significantly reducing the management problems stemming from exhausting or expanding DAS storage. DAS spillover emulates DAS management, giving users complete control to migrate to fabric-attached storage.

IT managers can also simplify data migration by using information life-cycle management (ILM) software to automate DAS spillover and offload reference data to fabric-attached storage. This frees up valuable direct-attached storage space and speeds access to transactional information in a process called cold data migration or adaptive storage migration.

Depending on the sophistication of the ILM software, this technique allows either block- or file-level data in direct-attached storage to be allocated to fabric-attached storage based on access frequency and application need. Adaptive data migration can be executed in a variety of ways. The user, for example, can initiate the migration manually or establish a schedule for migrating data automatically at regular intervals. IT managers also can set the ILM software to migrate data automatically in real time back and forth between the host and fabric and predict what data will be accessed next based on user-access patterns to optimize overall system performance.

DAS evacuation is the transparent and wholesale migration of DAS data to a fabric-attached storage device. The spillover region on the fabric-attached storage device must be expanded to accommodate the additional data. To the user, the operation is transparent and provides instant access to the new storage capacity. Evacuation decommissions DAS and moves the data to a fabric environment.

Today, most data centers use parallel storage technology like SCSI, but as requirements grow for increased performance, higher scalability needs and improved reliability, IT managers are looking to new serial technologies such as serial-attached SCSI. IT managers needing a simple, cost-effective way to migrate from parallel technology to serial-based systems with minimal disruption to business operations will find DAS spillover, adaptive storage migration and DAS evacuation powerful tools for enabling this transition.

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