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Chapter 11. Backup and Recovery

Data Backup

After accumulating a few petabytes of data or so, someone inevitably asks how all this data is going to be backed up. It’s a deceptively difficult problem to overcome when working with such a large repository of data. Overcoming even simple problems like knowing what has changed since the last backup can prove difficult with a high rate of new data arrival in a sufficiently large cluster. All backup solutions need to deal explicitly with a few key concerns. Selecting the data that should be backed up is a two-dimensional problem, in that both the critical datasets must be choosen, as must, within each dataset, the subset of the data that has not yet been backed up. The timeliness of backups is another important question. Data can be backed up less frequently, in larger batches, but this affects the window of possible data loss. Ratcheting up the frequency of a backup may not be feasible due to the incurred overhead. Finally, one of the most difficult problems that must be tackled is that of backup consistency. Copying the data as it’s changing can potentially result in an invalid backup. For this reason, some knowledge of how the application functions with respect to the underlying filesystem is necessary. Those with experience administering relational databases are intimately aware of the problems with simply copying data out from under a running system.

The act of taking a backup implies the execution of a batch operation that (usually) ...

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