Protecting the data
- Article 1 of 2
- Business Continuity, March 2004
Effective data back up and recovery processes are fundamental to a successful business continuity strategy.
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“It's not cheap, because you need to double everything,” says Miles Cunningham of SunGard Availability Services. “But you can get a gigabit circuit halfway across the UK for as little as GBP 50-60K a year.” An alternative, at the lower end, is to use a back-up service provider, such as Imperidata, which is able to 'trickle' a disk drive update to a remotely hosted copy over a standard broadband Internet connection without the user noticing.
Using remote replication, disk drives can automatically be brought online. And the second servers do not need to be reconfigured because the network address of the primary server can be virtualised using Cisco's hot standby router protocol (HRRP) or the international standard virtual router redundancy protocol (VRRP). A router at the apparent network address of the primary server forwards traffic to its usual address until the server fails, but redirects traffic to the second server in the event of a failure.
Virtualisation technology, whether of the network or of storage, has provided users with much more resilient systems all round. In the event of storage failure, for example, traffic from the still-functioning servers is simply redirected to the back-up network storage without any interruption in service.
When SANs were first introduced - during the 1990s - they solved some continuity problems but created others. Remote backing up of SANs was initially difficult: many SANs are based on fibre channel networking rather than standard Ethernet-based networking; they also often require expensive and proprietary virtualisation software, even for local back ups, to create easily manageable back-up processes.
This, however, has become easier. The advent of the iSCSI protocol, which forgoes the fibre channel of traditional SANs in favour of Gigabit Ethernet, has made it as easy to push iSCSI-based SAN data out over a leased line as it is to push out data from attached storage. “You're better off deploying a SAN if you want to do remote back ups,” says Stephen Owens, EMEA product manager of Adaptec, a storage systems supplier. “It's easier, you get the benefits of separate storage - you don't impact the LAN - and you can deploy snapshotting, remote mirroring and storage virtualisation.”
Quantum's Hunt concurs, adding that iSCSI will go where fibre channel can't. “Fibre channel isn't designed for long distances, but Ethernet has an unlimited range,” he says.
Remote back-up solutions face a barrier that local solutions do not face, however: the speed of light. Once servers are further apart than roughly 16 kilometres, there are noticeable lags in communication between primary and secondary sites that can affect performance, as messages confirming arrival and requests for more data are passed back and forth. With many companies now worried about large scale terrorist or natural disasters, this problem has moved from theoretical to practical.
Various storage firms claim to have overcome the problem, including EDS and Hitachi Data Systems. “We offer an asynchronous solution that can overcome the lag,” says John Hickman, business continuity manager, EMEA, HDS. Rather than write to both primary and secondary server simultaneously, asynchronous back up allows for lags of a few seconds before data is successfully written to the secondary server. “When we send out the data, we include metadata that specifies the order in which disk operations should be performed. The secondary server won't perform the writes until it knows it has received all the data.”
This also prevents disk corruption in the secondary server, since it will hold off writing data that is broken off by a breakdown in the primary server.
“It is not uncommon for some organisations to synchronously mirror their data to a secondary site 10 kilometres away,” agrees Paul Hammond, director of solutions consulting at CNT.“ Some then back up asynchronously to a third site, which could be up to 100 kilometres away.
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