StoreAge touts snapshot capabilities

Posted on October 01, 2003

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By Heidi Biggar

Analysts say that the way StoreAge Networking Technologies performs snapshots, in conjunction with its out-of-band storage virtualization, is a key differentiator for the three-year-old storage management software vendor. Last month, the company announced an enhanced mirroring capability that integrates with its MultiView snapshot software.

"The way that StoreAge uses snapshots to underlie a lot of other capabilities is unique," says Arun Taneja, founder and consulting analyst with The Taneja Group consulting firm, in Hopkinton, MA.

MultiView freezes the primary volume and writes changes to a new temporary location. All read I/Os are directed to the original frozen volume, all writes to a small temporary space. This compares to traditional copy-on-write techniques, where new blocks of data are written to the primary volume and changed data is moved to temporary snapshot areas.

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The advantage of the StoreAge approach versus the copy-on-write technique is that it not only creates read-writable snapshots, which can be used for testing purposes or to make snapshots of snapshots for various applications, but it minimizes the amount of disk capacity that is actually consumed during the snapshot process to just 256MB initially. When the aggregate write volume to the temporary space exceeds 256MB, capacity in chunks equal to 5% of the original source volume capacity is allotted.

"Our approach minimizes the storage space that is needed and it allows users to make snapshots of snapshots, which is particularly important in testing environments," says Mark Spowart, president of StoreAge. "Users end up with a snapshot with a writable volume, which means they can do a snapshot of a snapshot of a snapshot, etc."

To restore data in the event of data corruptions, users would go back to a previous point-in-time snapshot of the data, create a virtual volume from that volume, and then mount that volume to the application, explains Spowart.

What's unique about StoreAge's approach isn't the fact that it can create read-write snapshots, but it can do so using much less storage capacity, explains Taneja. For example, to make its snapshots writable, EMC has users make a business continuance volume (BCV), or multiple BCVs, of their snapshots.

Another thing that was unique about the StoreAge product—until EMC announced SRDF/A—is that it doesn't make you transmit every write, says Taneja. "Let's say that you change a block at T0 and you change it again at T1. Why would you want to transmit the data twice? Why not just transmit the latest one?" StoreAge allows you to do that, without affecting the write-order fidelity, he says.

EMC in August announced SRDF/A, an asynchronous version of its SRDF replication software. In addition to allowing users to replicate data (e.g., snapshots) over longer distances, the asynchronous version lowers replication costs.

EMC SRDF and SRDF/A are integrated with both EMC Snap and TimeFinder. Like other vendors' replication/snapshot products (e.g., Hitachi TrueCopy/Shadow Image and IBM PPRC/Flash Copy), the software, because it is not enabled by virtualization software, does not support any-to-any device or host connectivity.

EMC SRDF/A, for example, currently only works with EMC Symmetrix. EMC and Hitachi also only allow users to make a finite number of snapshots (16 and 9, respectively), while both IBM and StoreAge allow an infinite number.

An alternative to making low-capacity snapshots is to make full-size snapshots where each snapshot is the same size as the original volume or to make split-mirror snapshots that maintain a mirror of the source volume. Both consume significantly more capacity (see figure). Hitachi and IBM both make full-size snapshots.

StoreAge implements MultiView and other applications in an out-of-band (off-the-network) storage virtualization platform. Unlike in-band virtualization products from vendors such as DataCore, FalconStor, and Fujitsu Softek, the StoreAge Storage Virtualization Manager (SVM) appliance sits out of the data path, where it centrally manages pooled heterogeneous resources within a storage area network (SAN) and does not drain SAN bandwidth.

StoreAge MultiView is integrated with MultiMirror to provide a low-cost, low-capacity disaster-recovery product. After mirroring data to a local or remote location over IP using MultiMirror, MultiView is invoked to take low-capacity snapshots of the data, which can then be used for a variety of tasks, including backup and restore, application testing, and data warehouse updates.

New Fibre Channel/IP routing capabilities built into the SVM appliance allow users to mirror or replicate data over long distances; no third-party routers or other devices are required.

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