The SmartDedupe software module deduplicates data by scanning an Isilon cluster for identical data blocks. Each block is 8 KB. If SmartDedupe finds duplicate blocks, SmartDedupe moves a single copy of the blocks to a hidden file called a shadow store. SmartDedupe then deletes the duplicate blocks from the original files and replaces the blocks with pointers to the shadow store.
Deduplication is applied at the directory level, targeting all files and directories underneath one or more root directories. SmartDedupe not only deduplicates identical blocks in different files, it also deduplicates identical blocks within a single file.
You can first assess a directory for deduplication and determine the estimated amount of space you can expect to save. You can then decide whether to deduplicate the directory. After you begin deduplicating a directory, you can monitor how much space is saved by deduplication in real time.
For two or more files to be deduplicated, the files must have the same disk pool policy ID and protection policy. If one or both of these attributes differs between two or more identical files, or files with identical 8K blocks, the files are not deduplicated.
Because it is possible to specify protection policies on a per-file or per-directory basis, deduplication can further be impacted. Consider the example of two files, /ifs/data/projects/alpha/logo.jpg and /ifs/data/projects/beta/logo.jpg. Even though the logo.jpg files in both directories are identical, if one has a different protection policy from the other, the two files would not be deduplicated.
In addition, if you have activated a SmartPools license on your cluster, you can specify custom file pool policies. These file pool polices might cause files that are identical or have identical 8K blocks to be stored in different node pools. Consequently, those files would have different disk pool policy IDs and would not be deduplicated.
SmartDedupe also does not deduplicate files that are 32 KB or smaller, because doing so would consume more cluster resources than the storage savings are worth. The default size of a shadow store is 2 GB. Each shadow store can contain up to 256,000 blocks. Each block in a shadow store can be referenced up to 32,000 times.