IBM XIV STORAGE SYSTEM, User Manual

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Name Type Description Mandatory Default
name Object name Names of the new
snapshots. N Auto-generated
names.
delete_
priority
Integer The deletion
priority of the
volume's snapshot. N 1
overwrite Object name Name of an
existing snapshots
to be overwritten
with the current
volumes content. N N/A
This command creates a new snapshot for an existing volume, which is referred to
as the snapshot's master volume. The snapshot's content is the same as the master
volume at the exact point in time that the snapshot was created. The snapshot
remains unchanged, although the master volume keeps changing after the
snapshot is created. Upon a successful completion of this command, the snapshot
is created and assigned a name that can later be used by other commands. The
name doesn't have to be new. It could be of an already existing snapshot (in such a
case, the already existing snapshot is overridden).
A write operation can be processed at the exact time of the snapshot creation,
meaning that the write operation request was sent to the system before the
command was executed, while the write was acknowledged after the command
was executed. In this case, the content of the snapshot is not deterministic and may
either contain the original value before the write operation or the new value after
the write operation. In fact, the snapshot's data may even contain a mixture of the
two, where some blocks are equal to the volume before the write operation and
other blocks are equal to the value after the write operation.
The new snapshot is initially locked for changes.
The snapshot that is created acts like a regular volume, except for the differences
described below:
v The snapshot's name is either automatically generated from its master volume's
name or given as a parameter to the command. It can later be changed without
altering the snapshot's modification state.
v
Upon successful completion of the command, the system assigns a unique SCSI
ID to the snapshot. The creation time of the snapshot is set to the current time
and is never changed until the snapshot is deleted.
v
The size of the snapshot is the same as its master volume's size, but no storage
space is reserved for the snapshot. This means that the functionality of the
snapshot is not guaranteed. When the snapshot's Storage Pool is exhausted, the
snapshot may be deleted.
v The snapshot's lock state is initially set to "locked", and as long as it is not
"unlocked", the snapshot remains an exact image of the master volume at
creation time and can be the source for a restore operation. The modification
state of the snapshot is initially set to "unmodified".
During creation, the snapshot's deletion priority can be set explicitly, or it is
automatically set to the default value. The deletion priority determines which
snapshots will be deleted first when the Storage Pool runs out of snapshot storage.
Chapter 5. Volume Snapshot Management
65