Thin Image and Thin Image Advanced enables rapid creation of in-system, space efficient, read/write, volume-consistent snapshots and subsequent rollback of entire volumes.
When handling multiple primary volumes, the storage system takes a snapshot of each volume sequentially. This means that a slight difference can be seen across the snapshot timestamps. If exactly the same timestamp is required among the snapshot set (for crash- consistent backup), the snapshot set should be created using Consistency Groups (CTGs).
When taking a snapshot, there are two options for the target volumes:
- Differential Snapshot: Creates a new snapshot for each backup and deletes it when the retention time expires. This simplifies the management of numerous snapshots and is suitablefor backup operations. Data Retention Utility (DRU) protection can be applied to the snapshot's LDEV so that it can be used as a read-only volume or to protect it against both read and write operations.
- Refreshed Snapshot: Creates a new snapshot for the first backup, and then resynchronizes it on the following backups. This enables static target volumes (Port, Host Groupand LUN, although the LUN may not remain constant depending on the mount host OS) and is suitable for repurpose operations.
Floating Device is an improved snapshot capability, used in conjunction with Thin Image andThin Image Advanced, that simplifies snapshot management. With this capability snapshots can be created without creating target volumes upfront. This means that the limit on the number of snapshots in the entire storage system is increased (the number of snapshots of a specific primary volume is 1024). To revert the snapshot it is only necessary to select the required timestamp. To mount the snapshot for re-purposing, it must be mapped to a specific LDEV/LUN. After the snapshot is un-mounted, the volumes will be deleted by Protector as part of the unmount process.
The storage system supports two types of snapshots. Thin Image and Thin Image Advanced, the storage system will automatically decide the type of snapshot used. For Data Reduction Shared (DRS) volumes Thin Image Advanced snapshots will always be used and for all other types of volumes Thin Image snapshots will be used.
About Thin Image Snapshots:
Snapshots of a volume in the storage system are stored in a dedicated area called the Thin Image pool. For floating snapshots, where no LDEV is assigned until it is mounted, no data movement occurs and thus creation of a snapshot is near instantaneous. For non-floating snapshots, the auxiliary tasks of creating an LDEV and a LUN will take some time.
Once a snapshot is created, subsequent updates to the primary data causes the storage system to move the data blocks being updated to the TI pool. TI pool usage thus increases as the primary data changes and snapshots are retained.
Filling a Thin Image pool to capacity will invalidate all snapshot data contained within that pool. All snapshots in the pool will have to be deleted before snapshotting can be resumed.
When accessing a snapshot, the storage system presents the virtualized contents by merging the primary volume with its differentials held in the TI pool.
When deleting a snapshot, the storage system releases the differentials held in the TI pool. When multiple snapshots are involved, the delete operation may take some time. The differentials are reference counted and will only be deleted when no remaining snapshot requires them.
When handling multiple primary volumes, the storage system takes a snapshot of each volume sequentially. This means that a slight difference can be seen across the snapshot timestamps. If exactly the same timestamp is required among the snapshot set (for crash-consistent backup), the snapshot set should be created using Consistency Groups (CTGs).
When taking a snapshot, there are two options for the target volumes:
- Differential Snapshot: Creates a new snapshot for each backup and deletes it when the retention time expires. This simplifies the management of numerous snapshots and is suitable for backup operations. Data Retention Utility (DRU) protection can be applied to the snapshot's LDEV so that it can be used as a read-only volume or to protect it against both read and write operations.
- Refreshed Snapshot: Creates a new snapshot for the first backup, and then resynchronizes it on the following backups. This enables static target volumes (Port, Host Group and LUN, although the LUN may not remain constant depending on the mount host OS) and is suitable for repurpose operations.
Floating Device is an improved snapshot capability, used in conjunction with Thin Image, that simplifies snapshot management. With this capability snapshots can be created without creating target volumes upfront. This means that the limit on the number of snapshots in the entire storage system is increased (the number of snapshots of a specific primary volume is 1024). To revert the snapshot it is only necessary to select the required timestamp. To mount the snapshot for re-purposing, it must be mapped to a specific LDEV/LUN. After the snapshot is un-mounted, the volumes will be deleted by Protector as part of the unmount process.
About Thin Image Advanced Snapshots:
Snapshots of a volume in the storage system are stored in the same pool as the primary. When a snapshot is taken the data mapping of the primary volume is copied. Once a snapshot is created, subsequent updates to the primary data cause the storage system to redirect the new data blocks to new storage in the primary pool. Primary pool usage will increase as the primary data changes and snapshots retain the old data blocks.
- When using a snapshot, intensive read/write access to the snapshot may impact the performance of the primary volume, due to the way the snapshot volume is virtualised. If this is of concern then consider using ShadowImage or ShadowImage-Thin Image in cascade, where the Thin Image primary volume becomes the ShadowImage secondary volume instead of the original source.
- Long-term retention increases the number of differentials held in the TI pool once writes are distributed across all data blocks. Thus, for long-term backups, it is recommended to use ShadowImage.
- Thin Image requires the primary data in order to present a virtualised snapshot volume. This means that snapshots will be lost if a disk failure occurs on the primary volumes. To protect the data from such hardware failure use ShadowImage to create a clone and take snapshots of the clone instead.