eG Monitoring
 
Measures reported by NAUSDSnapshotsTest

A Snapshot copy is a point-in-time file system image. Low-overhead Snapshot copies are made possible by the unique features of the WAFL® (Write Anywhere File Layout) storage virtualization technology that is part of Data ONTAP®. Like a database, WAFL uses pointers to the actual data blocks on disk, but, unlike a database, WAFL does not rewrite existing blocks; it writes updated data to a new block and changes the pointer. A NetApp Snapshot copy simply manipulates block pointers, creating a “frozen” read-only view of a WAFL volume that lets applications access older versions of files, directory hierarchies, and/or LUNs (logical unit numbers) without special programming.

Whenever a volume/LUN/aggregate runs out of space, you may want to clear some space in that storage device so that there is no road-block to freely reading from and writing data into that device. To make room in a volume/LUN/aggregate for more data, you can start by deleting some snapshot copies from that storage device. Before attempting deletion however, you may want to determine the following:

  • How much space is occupied by each snapshot on the storage system?
  • Which snapshot copies, when deleted, will free more space? Will deleting the complete snapshot series make more space available?
  • Which snapshot copies are way too old, and are hence ideal candidates for deletion?
  • Which snapshots are easier to delete? - the snapshots containing LUN clones may take longer to delete as the LUN clones will first have to be deleted and then the snapshots.

Using this test, you can find quick and accurate answers for the questions above. This test auto-discovers the snapshot copies on the storage system, and for each snapshot copy, reports the space used by the snapshot copy, the age of the copy, and whether the copy contains LUN clones or not. Deletion decisions can be taken based on the insights provided by this test.

The measures made by this test are as follows:

Measurement Description Measurement Unit Interpretation
Total_blocks Indicates the percentage of blocks that were owned by this snapshot with respect to the total number of blocks in this volume. Percent Using this measure, large snapshots can be identified easily and helps you to decide whether this snapshot can be deleted so that the snapshot reserve space can be reclaimed.
Used_blocks Indicates the percentage of blocks that were owned by this snapshot with respect to the number of blocks that were currently used in this volume. Percent If a high percentage of used space in a volume/LUN/aggregate is in fact used up by a snapshot, then deleting such a snapshot can instantly reduce the space usage in that volume/LUN/aggregate, thereby enabling that storage medium to accommodate more data.

Compare the value of this measure across snapshots to identify the snapshot that is occupying a lot of used space.

Total_blocks_in_volume Indicates the percentage of total blocks that were owned by the snapshot series (including this snapshot) with respect to the total number of blocks in this volume. Percent Comparing the value of this measure across snapshots will instantly reveal the large-sized snapshots. These snapshots, when deleted, will release a large amount of snapshot reserve space.
Used_blocks_in_volume Indicates the percentage of total blocks that were owned by the snapshot series (including this snapshot) with respect to the total number of blocks that were currently used in this volume. Percent If the containing volume is running out of space, then this measure is used to clearly indicate if too much of space is occupied by the snapshot series in this volume and helps you to identify the amount of space that can be reclaimed by deleting one or more snapshots from the snapshot series.
Age Indicates the number of days that have elapsed since this snapshot was created. Days Generally, the snapshots should not be older than two weeks. This measure helps you to identify the snapshots that are old enough to be deleted.
Contains_lun_clone Indicates whether/not this snapshot contains LUN clones.   Lun clones are editable copies of LUNs which are backed by a snapshot. These snapshots cannot be deleted without deleting the associated LUN clones that are referenced by the snapshot.

This measure reports the value Yes if LUN clones exist for a snapshot and No if the LUN clones do not exist for a snapshot. The corresponding numeric equivalents for the measures are as detailed in the table below:

Numeric Value Measure Value
0 No
1 Yes

Note:

By default, this measure reports the above-mentioned Measure Values while indicating whether/not this snapshot contains LUN clones. However, the graph of this measure will be represented using the corresponding numeric equivalents of the states as mentioned in the table above.