Most data centers now consider backup disk as the first source of recovery when something has failed. In reality backup disk should be your second point of recovery not your first. As Storage Switzerland discussed in a recent article "Protecting Applications From Storage System Failures" most data centers should not be counting on the backup process at all for mission critical recoveries.
There should be some other form of recovery technology in place that provides direct access to data and a smaller window of time between data protection captures. Disk backup should be the second step in a recovery process when something goes wrong with a high availability (HA) solution or an older point in time of the data set is needed. The disk backup can also be used for primary recovery of less critical systems but we think the number of applications and services that can sustain multi-hour recovery times is decreasing.
Tape should be looked at as the third tier of recovery, when a much older point in time of data is needed or when something goes wrong with the previous two recovery steps. Despite this, there is also a situation where tape should be considered as the primary backup and recovery point. Consider tape first when a very large data set needs to, and can be, transferred across a very high-speed connection.
Tape, if it can be sustained at full streaming performance is incredibly fast, faster than many disk based backup systems. An example of this might be a database environment with a large 200GB+ data set where tape can either be attached directly or via a fibre channel connection. A transfer directly from the database server to tape is often faster than to a backup class disk system. A case can be made that tape is all you need here since many database applications have some other form of availability in place for quick recovery. The purpose of the backup then is to create an archive of the database and to get that data off-site. Something that tape is well suited for.
Tape is ideal for this third tier of backup data thanks to the continued progress of the technology. The cost per GB, performance and now the reliability and shelf life of tape make it an ideal compliment to disk backup. It provides an alternate, offline storage method in case something goes wrong with disk media. In most environments, HA solutions should be the point of first restore, disk backup should be the point of second restore and tape backup should be the safety net. The challenge is that the integration between these three layers is lacking and is something we will discuss in our next entry.
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