Continously protecting data sounds like an extremely good idea that is totally impractical. We know it can be done; it's called mirroring and involves duplicating your disks and writing to two disks every time data is changed instead of to just one. Mirroring doubles disk costs for the mirrored data. It's very secure but very expensive.

Add in replication with remote mirroring for disaster recovery and the WAN costs just make everyday mirroring out of reach except for the richest of organisations and their most precious data. Toigo Partners International wrote in a May, 2003, report entitled Introducing Time-Addressable Storage, 'For each terabyte available to the host application nine or more terabytes of disk capacity are commonly required to support mirror-splits and replication.'

Revivio aims to change this perception by making continuous protection more affordable. It is still an enterprise play but costs are no longer astronomic. Think low millions instead of tens of millions. This is still an enterprise scheme; SMB readers will probably find it out of reach.

The Intel-based appliance runs TimeOS, Revivio's application. It connects to hosts via direct or SAN connection.

The basic idea is to suck data off a business's drives whenever it is written to disk and store a duplicate on a Revivio appliance. The data is time-stamped. If there is a disaster or other data loss incident you just go back to a second before the loss and restore the lost data.

This is a straightforward enough concept. But how does it work and why is it better - cheaper - than mirroring?

Time-addressable storage
Revivio says its appliance provides days, even weeks of protection, that uses less disk space than a mirroring scheme. It is an appliance that doesn't come with storage. Instead it uses drives from any vendor. These are used for block-level storage, meaning that the scheme works at below the file system level and isn't limited to supporting particular applications or databases.

Protected servers 'see' a new set of LUNs called TimeSafe volumes. Data is written to these volumes with a time stamp. If an administrator calls for a restore at a particular point in time then the appliance presents it as a new volume set, called a TimeImage. These are mounted by a host and checked for correctness. No actual data is being moved here. It's all done with pointers - not smoke and mirrors!

Normally a recovered database needs much checking because the action of recovery alters the underlying storage. This is not so with TimeImages and this can radically shorten recovery time and reduce the need for intermediate data copies, a feature of mirror-splits.

Once the TimeImage is known to be correct the validated point-in-time data is restored to the TimeSafe volumes, again without moving data, and they can stand in for the primary data set. The business application restarts. To synchronise the primary storage, the data and the TimeSafe volume a background process is run.

So, what we have here is an appliance which stores data, time-stamped data, and pointers to that data, a sort of map of the data. It can present different selections of the data by using differently timed pointers, all data before 10pm on the 24th July for example.

In effect, no, in truth, the data IS being mirrored, but it is being mirrored with a time stamp. (The TimeSAfe volume is seen as a mirror by a host.) This means that the Revivio version of mirroring, with its feature of recovery not altering the underlying data, is more efficient in recovery terms than existing mirror-split style mirroring.

You still need double the disk storage, slightly more in fact as you need to store the time stamps in a TimeStore database. What you don't need are the multitudinous intermediate copies, up to nine of them, needed by the mirror-split implementation of mirroring. Revivio says that 80 percent or so of this extra disk capacity can be recovered with its style of continuous data protection.

The Revivio scheme enables recovery in minutes rather than the hours involved with mirror-splits. It allows recovery to the second. Typical mirror-split granularity is to a number of hours. And it doesn't entail the use of so much disk hardware.

If you are spending millions on mirror-split protection then check out Revivio. Perhaps by using its style of mirroring those additional costs can go up in smoke.