Item	Thin Image (HTI)	ShadowImage (SI)
P-VOL physical failures such as hard disks.	P-VOL data cannot be guaranteed.	P-VOL data can be recovered using the S-VOL.
P-VOL logical failures such as data update errors or viruses.	P-VOL data can be recovered using the S-VOL.	P-VOL data can be recovered using the S-VOL.
Capacity required for backup.	Less capacity is required for backups because only differential data of the P-VOL is retained.1	More capacity is required for backup because all data in the P-VOL is retained.
Impact on P-VOL performance when accessing backed up data.	P-VOL performance is affected because data in the P-VOL is shared.2	P-VOL performance is not affected because the P-VOL and the S-VOL can be disconnected.
S-VOL or pool physical failures	If a physical failure occurs in a pool3, data in all S-VOLs that use the pool cannot be guaranteed.	If a physical failure occurs in an S-VOL, data in the S-VOL cannot be guaranteed.
Notes: For snapshot pairs. When a cloned pair is created, all data in the P-VOL is retained and more capacity is required for backup. For snapshot pairs. When a cloned pair is created, the P-VOL and the S-VOL can be separated and the P-VOL performance is not affected. When a pool is full (the depletion threshold is exceeded in a pool for which the capacity for TI pairs is limited), data in all S-VOLs using the pool cannot be guaranteed.

Recommended usage

To maintain backed up data for long periods, save it on magnetic tapes or other media. For temporary backups use HTI or SI. When backing up data to magnetic tapes use SI. To reduce the capacity necessary for backups use HTI, but note that this affects P-VOL performance.

Use SI to minimize the impact from P-VOL physical failures. If you need four or more generations of backups, use both SI and HTI as shown in the following figure.

Use HTI to minimize P-VOL logical failures.