16 Implementing the IBM Storwize V5000
Single-tiered storage pool
MDisks that are used in a single-tiered storage pool should have the following characteristics
to prevent performance and other problems:
򐂰 They should have the same hardware characteristics, for example, the same RAID type,
RAID array size, disk type, and disk revolutions per minute (RPMs).
򐂰 The disk subsystems providing the MDisks must have similar characteristics, for example,
maximum input/output operations per second (IOPS), response time, cache, and
򐂰 Use MDisks of the same size, and ensure that the MDisks provide the same number of
extents. If this configuration is not feasible, you must check the distribution of the volumes’
extents in that storage pool.
Multitiered storage pool
A multitiered storage pool has a mix of MDisks with more than one type of disk tier attribute,
for example, a storage pool that contains a mix of generic_hdd AND generic_ssd MDisks.
A multitiered storage pool contains MDisks with different characteristics unlike the
single-tiered storage pool. However, each tier should have MDisks of the same size and
MDisks that provide the same number of extents.
A multitiered storage pool is used to enable automatic migration of extents between disk tiers
by using the IBM Storwize V5000 Easy Tier® function, as described in Chapter 9, “Easy Tier”
on page 411.
1.5.9 Volumes
A volume is a logical disk that is presented to a host system by the clustered system. In our
virtualized environment, the host system has a volume that is mapped to it by IBM Storwize
V5000. IBM Storwize V5000 translates this volume into a number of extents, which are
allocated across MDisks. The advantage with storage virtualization is that the host is
decoupled from the underlying storage, so the virtualization appliance can move around the
extents without impacting the host system.
The host system cannot directly access the underlying MDisks in the same manner as it can
access RAID arrays in a traditional storage environment.
The following types of volumes are available:
򐂰 Striped
A striped volume is allocated one extent in turn from each MDisk in the storage pool. This
process continues until the space that is required for the volume is satisfied.
It also is possible to supply a list of MDisks to use.
Chapter 1. Overview of the IBM Storwize V5000 system 17
Figure 1-9 shows how a striped volume is allocated, assuming 10 extents are required.
Figure 1-9 Striped volume
򐂰 Sequential
A sequential volume is a volume in which the extents are allocated one after the other from
one MDisk to the next MDisk, as shown in Figure 1-10.
Figure 1-10 Sequential volume
18 Implementing the IBM Storwize V5000
򐂰 Image mode
Image mode volumes are special volumes that have a direct relationship with one MDisk.
They are used to migrate existing data into and out of the clustered system to or from
external FC SAN-attached storage.
When the image mode volume is created, a direct mapping is made between extents that
are on the MDisk and the extents that are on the volume. The logical block address (LBA)
x on the MDisk is the same as the LBA x on the volume, which ensures that the data on
the MDisk is preserved as it is brought into the clustered system, as shown in Figure 1-11.
Figure 1-11 Image mode volume
Some virtualization functions are not available for image mode volumes, so it is often useful to
migrate the volume into a new storage pool. After it is migrated, the MDisk becomes a
managed MDisk.
If you want to migrate data from an existing storage subsystem, use the Storage Migration
wizard, which guides you through the process.
For more information, see Chapter 6, “Storage migration wizard” on page 237.
If you add an MDisk that contains data to a storage pool, any data on the MDisk is lost. If you
are presenting externally virtualized LUNs that contain data to a IBM Storwize V5000, import
them as image mode volumes to ensure data integrity or use the migration wizard.

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