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Why do restores take longer than backups?

Why do restores take longer than backups on the same hardware configuration?
The performance variation between both operations is mostly attributable to the number of steps involved in each. Essentially, a backup is not much more that a read operation. Data is simply read from the backup client and sent to the backup server.

Conversely, a restore process implies a write operation. Because data is usually being written to a file system during a restore operation, it involves some file system overhead such as creating an entry in a file allocation table or journal, permission setting, ACL and various other file attributes being recorded. This obviously takes more system cycles and ends up being much slower than a simple backup (read) operation.

Some file systems are better than others are at this and a significant restore performance difference can be noted. However, generally speaking, the worst possible restore performance will be observed when restoring a large number (millions) of small files. In comparison, restoring a 500 GB database will be dramatically faster that restoring a 500 GB file server.

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