Dell PowerVault DP500 - Manual (Page 22)

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Operating System Features
Single Instance Storage
Single Instance Storage (SIS) recovers disk space by reducing the amount of
redundant data stored on a volume. To recover the disk space, SIS performs
the following procedures:
• Identifies identical files
• Stores one copy of the file in the SIS common store
• Replaces the files with pointers to the file in the SIS common store.
For example, user 1 and user 2 receive the same e-mail with an attachment. Both
users save the attachment to their home folder. SIS detects the two identical files
on the volume, moves one of the copies into the SIS common store, and then
replaces both files with a pointer to the file in the SIS common store.
Some time later, user 1 makes a change to the file. SIS immediately detects
that an update is pending for the file, removes the pointer in the user’s home
folder, and replaces the pointer with a copy of the file in the SIS common store.
Then, the updates are applied to a fresh copy of the original file. This process
is completely transparent to the application.
The file of user 2 remains in the SIS common store with a pointer in the user's
home folder, even if there is only one pointer to the file. When user 2 updates
the file (assuming there are no other pointers), the pointer is deleted and
replaced with a copy of the original file. The file in the SIS common store is
deleted.
SIS works in the background and does not require user intervention.
The administrator can enable SIS on a per volume basis. For best results, the
administrator should use a backup application that supports SIS.