Migrating from legacy storage with the minimum disruption requires a good strategy, support from the business and the right technology, such as storage virtualisation or the use of migration appliances.
Before carrying out any migration, it is wise to analyse the content of the data. Web documents, web pages and e-mails can constitute 80% of a company's data, with 50-60% of the documents stored being duplicates, says Nic Archer, senior vice-president of enterprise migration specialist Vamosa.
"By analysing your content - its current state and its quality - in advance, you can reduce the challenge of migration," he says. Data de-duplication can also bring significant cost savings.
As well as analysing the content, organisations can ease migration by carefully examining how applications are accessing the storage (for example files, databases and raw volumes), and by looking at the connectivity that is being used (direct attached, fibre channel SAN, or IP SAN).
"As well as the data movement itself, the other consideration in any migration project is the need to change the host device drivers and/or path management drivers to suit the new storage devices. This may in turn have dependencies on operating system levels," advises Steve Legg, CTO at IBM System Storage.
The secret to minimising disruption when migrating storage is to build the plan around migration events related to the storage that is linked to particular servers and applications, says storage specialist Tony Reid, UK services director at Hitachi Data Systems.
"A good plan will give a single application outage, keeping the rest online," he says.
Organisations looking to perform data migration face a Catch-22 situation, says Roger Llewellyn, chief executive officer of Kognitio, which has migrated data for the likes of Prudential and Bupa.
He explains that a "big bang" migration, where all data is migrated in a single action, requires rigorous testing and auditing, which can mean lengthy downtime of 10 days or more.
The alternative, migrating data piecemeal over a longer period, means that as systems are constantly changing and growing, the migration will need to continually adapt to these changes, taking longer and longer. "Either way, while a migration might be effective, the problems involved will become crippling," says Llewellyn.
One of the ways to reduce the testing time in big bang migrations is to use analysis and testing tools which use in-memory and massively parallel data processing techniques so tests can be conducted quickly, and migrations shortened to less than 48 hours in some cases.
Many industry experts argue that virtualisation brings the capability to migrate efficiently from legacy storage.
Sean Haffey, storage business manager at Fujitsu Technology Solutions, says that with virtualisation, migration to new technology, whether a new tape robot or disc, is handled quickly and transparently. "Payback is typically 12 to 15 months, which for a large project such as this is an exceptional schedule for ROI."
Nick Broadbent, European director at DataCore, which frequently carries out legacy data migrations, says problems are often caused by different manufacturers' storage subsystems not communicating well with each other.
However, storage virtualisation brings a new flexible world, says Broadbent. "Moving data from unlike storage, from legacy to new, can be done in place without disruption."
Appliance-based storage migration products such as IBM's SVC, DataCore and FalconStor, and application-level tools such as Softek's TDMF, have been available for a number of years, and many use storage virtualisation technology to move data and applications between storage arrays.
But there is a new breed of appliance-based products, which includes Incipient, HP's SVSP (based on LSI's SVM), HDS's USP and NetApps' Filer Gateway, says Mark Sweeney, technical architect at Logicalis UK.
"These allow a seamless migration between the older legacy arrays and the newer, current arrays, and also enable such migrations to occur without interruption to the business," he adds.
But Sweeney says it is important not to be dazzled by some of these new technologies, which can add cost and complexity and may still be in the early adopter stage.
"While it won't be the case forever, the use of tapes is still a popular and effective migration technique, offering a lower-risk, non-destructive and cheaper migration," he says.