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File System Snapshots
As data continues to grow and our backup windows continue to shrink, new technologies are needed to augment what we can do with the previously mentioned backup methods. In the latest UNIX versions, we have a strategy called file system snapshots.
A file system snapshot is a frozen image or picture of a file system at a given instant of time. Snapshots allow for many important features, including the abilities to provide:
- Backups of the file system at several times during the day without needing large amounts of additional storage media
- A way to perform file system integrity checks on a running and changing file system in an effort to reclaim lost blocks
- Perhaps most importantly, reliable off media backups without the need for long backup windows.
File system snapshots offer system administrators the freedom to create reliable backups of their systems without needing to shut down running applications for fear of data on disk changing while the backup is happening. Some vendor implementations of snapshots have the ability to mount this Point In Time Recovery (PITR) image as a read-only file system that you can then easily recover individual files from. According to Darcy Buskermolen, a network administrator from Wavefire Technologies:
“The frozen point in time image is extremely useful when it comes to being able to back up databases, allowing you to make a perfect backup of the database without needing any sort of maintenance window, thereby increasing your product/application accessibility uptime, as well as providing you with the reassurance that you are better protected from both system failure and user mishaps.
“I recently used snapshots to recover from a botched system upgrade that without them would have resulted in 100s of hours of customer downtime, a sleepless night spent restoring files from the full and incremental backups, and the usual barrage of phone calls complaining of downtime,” continues Buskermolen.
Volume Shadow Copy Technology in Windows Server 2003
Windows Server 2003 has introduced its own snapshot technology known as Volume Shadow Copy Technology (VSCT). The basic function of VSCT is threefold: applications can continue to write data to the volume during a backup; files that are open are no longer omitted during a backup; and backups can be performed at any time, without locking out users or having to worry about the backup window.
VSCT allows you to create shadow copy backups of volumes — exact point-in-time copies of files, including all open files. For example, databases that are continually held open and files that are open due to operator or system activity are backed up during a volume shadow copy backup. In this way, files that have changed during the backup window are copied correctly.
In this article, we have explored some of the strategies that can be employed to manage the process of backing up large amounts of data. In the second part of the storage basics series on backups, we'll move beyond these general methods of managing the backup window to more specific strategies, including using server-free and LAN-free backups, and we'll take a look at designing the network infrastructure to handle backup requirements. In addition, we will provide an overview of backup security in a SAN environment and revisit the debate of tape versus disk backups.