User`s guide

28 Chapter 2 Understanding Concepts and Terms
 Manually: The Administrator selects the organization and all other parameters described
above, and specifies the drives that can be used for the volume. The XStack Storage unit then
finds the storage space on the given drives to allocate and create the Volume.
Disk space is assigned to a volume in extents. The extents that make up a volume can be seen on
the XStack Storage Management console. The extents are organized into members. The definition
of a member varies with the organization:
 For a striped organization, a member is defined above.
 For a mirror organization, there are two members, one for each copy of the data.
 A stripe mirror organization has a member for each stripe, as defined above, plus a second
member (for each stripe) to hold the data copy. Therefore, if a stripe mirror organization has
x stripes, it has 2x members.
2.5 Spare Count
The XStack Storage Administrator can designate a spare count, which is a number of drives to be
kept available in case a drive that holds volume data fails. The spare count default is 0, meaning
that no drives are reserved. If the spare count was 1 in an enclosure with 12 drives installed, 11
drives could be used by the XStack Storage unit for automatic volume creation and one drive
would remain in the available pool. As volumes are created, drives would be moved from the
available pool into the pool in which the volume is created, up to 11 drives. Should any of those
drives fail, the 12
th
drive would become available to be used in any rebuild task that requires a
drive (the rebuild task is described in Section
2.6).
2.6 Tasks
The XStack Storage unit can automatically, or at the Administrator’s demand, perform activities
that take time and consume the controller’s resources. The Administrator can control, to some
degree, when tasks are performed. Any task can be suspended and resumed by the Administrator.
Some tasks can be cancelled and some can be scheduled on a recurring, periodic interval. All
tasks can have their priority changed, which controls the amount of resources the XStack Storage
unit devotes to a task.
The XStack Storage unit can perform the following tasks:
 Initialize a Volume: Some volume organizations (e.g., parity) require Initialization. The
Initialization task performs this action. This task can be performed while an initiator is
accessing (reading and writing) data. An Initialization task can be suspended and resumed,
but cannot be cancelled.
 Rebuild a Volume: When a drive fails, every redundant volume that occupied space on that
drive can be rebuilt.
– For mirror protection, data can be copied from the remaining copy.
– For parity protection, data can be recreated from the remaining data and parity
information.