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What deduplication metadata is

The deduplication metadata includes the fingerprint file and change logs. Fingerprints are the digital signatures for every 4-KB data block in a FlexVol volume.

The deduplication metadata contains two change log files. When deduplication is running, the fingerprints of the new data blocks from one change log file are merged into the fingerprint file, and the second change log file stores the fingerprints of the new data that is written to the volume during the deduplication operation. The roles of the change log files are reversed when the next deduplication operation is run.

In Data ONTAP 8.0.1, the deduplication metadata is located within the aggregate. Starting with Data ONTAP 8.1, two copies of deduplication metadata are maintained per volume. A copy of the deduplication metadata resides in the volume and another copy is in the aggregate. The deduplication metadata in the aggregate is used as the working copy for all the deduplication operations. An additional copy of the deduplication metadata resides in the volume.

When a volume is moved, the deduplication metadata is also transferred with the volume. If the volume ownership changes, the next time deduplication is run, then the deduplication metadata which resides in the aggregate is created automatically by using the copy of deduplication metadata in the volume. This method is a faster operation than creating a new fingerprint file.

Starting with Data ONTAP 8.2, the fingerprints are stored for each physical block, this reduces the amount of space required to store the deduplication metadata.

Deduplication metadata can occupy up to 7 percent of the total physical data contained within the volume, as follows:

You can use the df -A aggrname command to check the available space in an aggregate and the df path command to check the available space in a volume. For more information about these commands, see the man pages.

Example

A 2 TB aggregate has four volumes, each 400 GB in size, in the aggregate. You need three volumes to be deduplicated with varying savings percentage on each volume.

The space required in the different volumes for deduplication metadata is as follows:

The aggregate needs a total of 8.25 GB [(3% × (50% of 100 GB)) + (3% × (75% of 200 GB)) + (3% × (25% of 300 GB)) = 1.5+4.5+2.25= 8.25 GB] of space available in the aggregate for deduplication metadata.