Pruning previously-allocated free blocks from a synthetic backup

1. A non-transitory computer-readable medium storing a program that causes a processor to execute a method of pruning previously-allocated free blocks from an image-based synthetic backup, the method comprising:

Identifying multiple sequential image-based backups of source storage to be included in an image-based synthetic backup of the source storage, the multiple sequential image-based backup including a most recent image-based backup;

accessing a file system block allocation map (FSBAM) of the most recent image-based backup, the FSBAM indicating, as being free, block positions that were free in the source storage at a creation time of the most recent image-based backup;

identifying a set of allocated blocks that includes a most recent allocated block for each unique block position contained within the multiple sequential image-based backups;

pruning the set of allocated blocks to exclude at least some allocated blocks that correspond to block positions that are indicated as being free in the FSBAM, the excluded blocks including blocks that correspond to block positions that were previously allocated in the source storage prior to the creation time of the most recent image-based backup;

storing the pruned set of allocated blocks, and not storing the excluded allocated blocks, in the image-based synthetic backup, thereby decreasing the number of blocks that are stored in the image-based synthetic backup; and

storing a copy of the FSBAM with the synthetic backup.

2. A non-transitory computer-readable medium storing a program that causes a processor to execute a method of pruning previously-allocated free blocks from an image-based synthetic backup, the method comprising:

identifying multiple sequential image-based backups of a source storage to be included in an image-based synthetic backup of the source storage, the multiple sequential image-based backups including a most recent image-based backup;

accessing a file system block allocation map (FSBAM) of the most recent image-based backup that was stored with the most recent image-based backup, the FSBAM indicating, as being free, block positions that were free in the source storage at a creation time of the most recent image-based backup;

identifying a set of allocated blocks that includes a most recent allocated block for each unique block position contained within the multiple sequential image-based backups;

pruning the set of allocated blocks to exclude at least some allocated blocks that correspond to block positions that are indicated as being free in the FSBAM, the excluded blocks including blocks that correspond to block positions that were previously allocated in the source storage prior to the creation time of the most recent image-based backup;

storing the pruned set of allocated blocks, and not storing the excluded allocated blocks, in the image-based synthetic backup, thereby decreasing the number of blocks that are stored in the image-based synthetic backup; and

storing a copy of the FSBAM with the synthetic backup.

3. The non-transitory computer-readable medium as recited in claim 2 , wherein the multiple sequential image-based backups include an image-based base backup.

4. The non-transitory computer-readable medium as recited in claim 2 , wherein the multiple sequential image-based backups include an image-based synthetic base backup.

5. The non-transitory computer-readable medium as recited in claim 2 , wherein the multiple sequential image-based backups include an image-based synthetic incremental backup.

6. The non-transitory computer-readable medium as recited in claim 2 , wherein the FSBAM that was stored with the most recent image-based backup was stored as a separate data structure.

7. The non-transitory computer-readable medium as recited in claim 2 , wherein accessing the FSBAM of the most recent image-based backup includes mounting the most recent image-based backup as a virtual device and querying a file system of the virtual device to expose the FSBAM embedded in the mounted most recent image-based backup.

8. The non-transitory computer-readable medium as recited in claim 2 , wherein accessing the FSBAM of the most recent image-based backup includes parsing file system metadata contained within the most recent image-based backup to directly access the FSBAM that is embedded therein.

9. The non-transitory computer-readable medium as recited in claim 1 , wherein the multiple sequential image-based backups include only image-based incremental backups and/or image-based synthetic incremental backups.

10. The non-transitory computer-readable medium as recited in claim 1 , wherein the multiple sequential image-based backups include an image-based base backup and one or more image-based incremental backups.

11. The non-transitory computer-readable medium as recited in claim 1 , wherein the multiple sequential image-based backups include an image-based synthetic base backup and one or more image-based non-synthetic incremental backups.

12. The non-transitory computer-readable medium as recited in claim 1 , wherein the multiple sequential image-based backups include an image-based synthetic incremental backup and one or more image-based non-synthetic incremental backups.

13. The non-transitory computer-readable medium as recited in claim 1 , wherein accessing the FSBAM of the most recent image-based backup includes directly accessing the FSBAM that was stored with the most recent image-based backup as a separate data structure.

14. The non-transitory computer-readable medium as recited in claim 1 , wherein accessing the FSBAM of the most recent image-based backup includes mounting the most recent image-based backup as a virtual device and querying a file system of the virtual device to expose the FSBAM embedded in the mounted most recent image-based backup.

15. The non-transitory computer-readable medium as recited in claim 1 , wherein accessing the FSBAM of the most recent image-based backup includes parsing file system metadata contained within the most recent image-based backup to directly access the FSBAM that is embedded therein.

16. The non-transitory computer-readable medium as recited in claim 2 , wherein the multiple sequential image-based backups include only image-based incremental backups.