When you take a snapshot of a thin image (TI), the array creates a virtual volume (V-VOL) that contains the snapshot. Until the next write operation on the primary volume (P-VOL) device, V-VOL snapshots share data blocks with the P-VOL devices.
Because read operations on the snapshots might affect the performance of the P-VOL devices, and a failure on the P-VOL device can corrupt the V-VOL device, the Hitachi Vantara Command Control Interface (CCI) engine uses clones as the primary source of snapshots instead of using P-VOL devices, as a way to overcome these drawbacks that sharing the data blocks has.
In Hitachi Vantara terms, a clone is a ShadowImage (SI) clone and a volume that contains an SI clone is a secondary volume (S-VOL) device. The snapshot of clone feature is a TI snapshot of an SI clone (that is, a snapshot of an S-VOL device).
The Hitachi Vantara VSP using Targetless CCI Engine does not support the snapshot of clone feature.
Naming the Secondary Volume (S-VOL)
During the first snapshot backup operation, the first available mirror volume is selected, and then labeled IS_SVOL_CASCADE. That is the S-VOL volume. Subsequent TI snapshot backup jobs use the IS_SVOL_CASCADE volume for snapshot operations like mount, and backup copy.
When the S-VOL is selected, it is synchronized with the P-VOL, and then a TI snapshot is taken of the S-VOL. The SI remains in a suspended (PSUS) state until the next snapshot backup operation. The IS_SVOL_CASCADE volume is never deleted, even if the TI snapshots are deleted.
Notes:
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The IS_SVOL_CASCADE volume is never deleted, even if the TI snapshots are deleted.
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The revert operation is not supported.
