IBM Storwize V5020 User Manual

28 Implementing the IBM Storwize V5000 Gen2 with IBM Spectrum Virtualize V8.1
When a host system issues a write to a mirrored volume, IBM Storwize V5000 Gen2 writes
the data to both copies. When a host system issues a read to a mirrored volume, IBM
Storwize V5000 Gen2 requests it from the primary copy.
If one of the mirrored volume copies is temporarily unavailable, the IBM Storwize V5000 Gen2
automatically uses the alternative copy without any outage for the host system. When the
mirrored volume copy is repaired, IBM Storwize V5000 Gen2 synchronizes the data again.
A mirrored volume can be converted into a non-mirrored volume by deleting one copy or by
splitting away one copy to create a non-mirrored volume.
The use of mirrored volumes can also assist with migrating volumes between storage pools
that have different extent sizes. Mirrored volumes can also provide a mechanism to migrate
fully allocated volumes to thin-provisioned or compressed volumes without any host outages.
The Volume Mirroring feature is included as part of the base software, and no license is
required.
1.7.2 Thin provisioning
Volumes can be configured to be thin-provisioned or fully allocated. A thin-provisioned
volume behaves as though it were a fully allocated volume in terms of read/write I/O.
However, when a volume is created, the user specifies two capacities: the real capacity of the
volume and its virtual capacity.
The
real capacity determines the quantity of MDisk extents that are allocated for the volume.
The
virtual capacity is the capacity of the volume that is reported to IBM Storwize V5000
Gen2 and to the host servers.
The real capacity is used to store the user data and the metadata for the thin-provisioned
volume. The real capacity can be specified as an absolute value or a percentage of the virtual
capacity.
The Thin Provisioning feature can be used on its own to create over-allocated volumes, or it
can be used with FlashCopy. Thin-provisioned volumes can be used with the mirrored volume
feature, also.
A thin-provisioned volume can be configured to
auto expand, which causes the IBM Storwize
V5000 Gen2 to automatically expand the real capacity of a thin-provisioned volume as it gets
used. This feature prevents the volume from going offline. Auto expand attempts to maintain a
fixed amount of unused real capacity on the volume. This amount is known as the
contingency capacity.
When the thin-provisioned volume is initially created, the IBM Storwize V5000 Gen2 initially
allocates only 2% of the virtual capacity in real physical storage. The contingency capacity
and auto expand features seek to preserve this 2% of free space as the volume grows.
If the user modifies the real capacity, the contingency capacity is reset to be the difference
between the used capacity and real capacity. In this way, the autoexpand feature does not
cause the real capacity to grow much beyond the virtual capacity.
A volume that is created with a zero contingency capacity goes offline when it must expand.
A volume with a non-zero contingency capacity stays online until it is used up.
To support the auto expansion of thin-provisioned volumes, the volumes themselves have a
configurable warning capacity. When the used free capacity of the volume exceeds the
warning capacity, a warning is logged.

IBM Storwize V5020 Specifications

General

Brand	IBM
Model	Storwize V5020
Category	Storage
Language	English