To reduce disk fragmentation, space is always allocated for the first backup of a given session in 1 GB increments, until the session ends. If four streams are writing, each stream uses a pre-allocated chunk of disk space.
The last 1 GB chunk of data is rounded down so that the data file occupies the actual compressed session size after the deduplication process is complete. This approach helps to ensure that the disk is fragmented in 1 GB chunks.
This is done only for backups to deduplication devices and only for the first backup of one root directory on one device.
For second backup and subsequent jobs, the amount of data physically written to disk is expected to be low.
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