Specifications

NetApp Deduplication for FAS and V-Series Deployment and Implementation Guide
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4.2 DEDUPLICATION AND SNAPRESTORE
The SnapRestore
®
functionality is supported with deduplication, and it works in the same way with
deduplication as it does without deduplication. If you’re running Data ONTAP 7.3, note the following.
Starting with Data ONTAP 7.3, the deduplication metadata files (the fingerprint database and the change log
files) do not get restored when SnapRestore is executed, because they are located outside the volume in
the aggregate. In this case, after the SnapRestore operation, there is not a fingerprint database file in the
active file system for the data. This data, however, retains the original space savings. After SnapRestore, if
deduplication is enabled on the volume, any new data written to the volume continues to be deduplicated.
However, the deduplication process obtains space savings in the new data only and does not deduplicate
between the new data and the restored data. To run deduplication for all the data in the volume (and thus
obtain higher space savings), use the sis start -s command. This command builds the fingerprint
database for all the data in the volume. Depending on the size of the logical data in the volume, this process
can take a long time to complete.
Before using the sis start -s command, make sure that both the volume and the aggregate containing
the volume have sufficient free space to accommodate the addition of the deduplication metadata. For
information about how much extra space to leave for the deduplication metadata, see the section
―Deduplication Metadata Overhead.‖
4.3 DEDUPLICATION AND THE VOL COPY COMMAND
When deduplicated data is copied by using the volume copy command, the copy of the data at the
destination location inherits all the deduplication attributes and storage savings of the original data.
Starting with Data ONTAP 7.3, some of the deduplication metadata files do not get copied by the vol copy
command, because they are located outside of the volume in the aggregate. In this case, there is no
fingerprint database file in the destination volume for the data. However, the data retains the space savings.
The deduplication process also continues for any new data written to the destination volume and creates the
fingerprint database for the new data. The deduplication process obtains space savings in the new data only
and does not deduplicate between the new data and the old data. To run deduplication for all the data in the
cloned volume (and thus obtain higher space savings), use the sis start -s command. Depending on
the size of the logical data in the volume, this process can take a long time to complete.
4.4 DEDUPLICATION AND READ REALLOCATION (REALLOC)
For workloads that perform a mixture of random writes, and large and multiple sequential reads, read
reallocation improves the file layout and the sequential read performance. When you enable read
reallocation, Data ONTAP analyzes the parts of the file that are read sequentially. If the associated blocks
are not already largely contiguous, Data ONTAP updates the file layout by rewriting those blocks to another
location on disk. The rewrite improves the file layout, thus improving the sequential read performance the
next time that section of the file is read. However, read reallocation might result in more storage use if
Snapshot copies are used. It might also result in a higher load on the storage system. If you want to enable
read reallocation but storage space is a concern, you can enable read reallocation on FlexVol
®
volumes
using the space_optimized option. The space_optimized option conserves space but can slow read
performance through the Snapshot copies. Therefore, if fast read performance through Snapshot copies is a
high priority to you, do not use space_optimized.
A read reallocation scan does not rearrange blocks on disk that are shared between files by deduplication
on deduplicated volumes. Since read reallocation does not predictably improve the file layout and the
sequential read performance when used on deduplicated volumes, performing read reallocation on
deduplicated volumes is not supported. Instead, for files to benefit from read reallocation, they should be
stored on volumes that are not enabled for deduplication.
4.5 DEDUPLICATION AND FLEXCLONE VOLUMES
When a FlexClone volume (cloned volume) is created:
The FlexClone volume of a deduplicated volume is a deduplicated volume.