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Should we use disk-to-disk backup?

We are implementing a critical application over SAN. We estimate to have around 2.7 TB data to be stored on SAN and that the data will grow by 40 GB everyday. Currently, our SAN can store data up to 4.5 TB. We want to economically back up the data. We are thinking about backing up the data on the SAN itself, using disk-to-disk. My questions are:

a. Is there a more economical way to backup the SAN data?
b. Is disk-to-disk SAN backup a trusted way to back up the SAN?
c. What care needs to be taken while implementing this technology?

Depending on the type of SAN disk used in your environment, there might be a more economical way to backup your data; some tape technologies are still cheaper than high-end SAN based disk storage. That being said, you mention "a critical application" at the beginning of your question which puts "economy" in a different perspective. When designing a backup solution for a critical application, the first criteria should always be business requirements; cost should be considered after. For example, if your 2.7 TB application has a recovery time objective (RTO) of 8 hours, the possibly more economical (and slower) tape backup is definitely not the best solution.

As far as reliability, this depends on what you consider disk-to-disk backup over the SAN. If you mean data replication over the SAN, this method has been around for a while and is proving to be increasingly reliable. However, some feel the FC protocol still lacks the robustness of TCPIP based when it comes to error handling during data transfer. On the other hand, if you are referring to a backup software solution capable of backing up to a disk device that is Fibre (SAN) attached, that technology has been around for a long time (arguably since the mainframe days) and is definitely reliable.

With respect to the last part of your question, there are few different ways to implement disk-based backups. Some are hardware specific and some software based. A few vendors now offer "virtual tape library" solutions where low cost SATA disk storage appears as tape devices to the backup software. These solutions integrate with leading backup software products without requiring special modules or configuration changes. On the other hand, they do require the purchase of specific hardware components. Other solutions are based on the backup software's ability to use disk devices (on or off the SAN) as backup media. These solutions are typically not hardware centric thus allowing the use of existing SAN disk storage or lower cost disk such as SATA.

In any case, off-site backup data storage capabilities must be considered when contemplating any type of disk-to-disk backup design.

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