IBM DS8880 Series, Introduction And Planning Manual

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Physical and effective capacity
Use the following information to calculate the physical and effective capacity of a
storage system.
To calculate the total physical capacity of a storage system, multiply each drive-set
feature by its total physical capacity and sum the values. For the standard drive
enclosures, a full drive-set feature consists of 16 identical disk drives with the same
drive type, capacity, and speed. For High Performance Flash Enclosures Gen2,
there are 16 identical flash drives.
The logical configuration of your storage affects the effective capacity of the drive
set.
Specifically, effective capacities vary depending on the following configurations:
RAID type and spares
Drives in the DS8000 must be configured as RAID 5, RAID 6, or RAID 10
arrays before they can be used, and then spare drives are assigned. RAID
10 can offer better performance for selected applications, in particular, high
random, write content applications in the open systems environment.
RAID 6 increases data protection by adding an extra layer of parity over
the RAID 5 implementation.
Data format
Arrays are logically configured and formatted as fixed block (FB) or count
key data (CKD) ranks. Data that is accessed by open systems hosts or
Linux on IBM Z that support Fibre Channel protocol must be logically
configured as FB. Data that is accessed by IBM Z hosts with z/OS or
z/VM must be configured as CKD. Each RAID rank is divided into
equal-sized segments that are known as extents.
The storage administrator has the choice to create extent pools of different
extent sizes. The supported extent sizes for FB volumes are 1 GB or 16 MB
and for CKD volumes it is one 3390 Mod1, which is 1113 cylinders or 21
cylinders. An extent pool cannot have a mix of different extent sizes.
On prior models of DS8000 series, a fixed area on each rank was assigned to be
used for volume metadata, which reduced the amount of space available for use
by volumes. In the DS8880 family, there is no fixed area for volume metadata, and
this capacity is added to the space available for use. The metadata is allocated in
the storage pool when volumes are created and is referred to as the pool overhead.
The amount of space that can be allocated by volumes is variable and depends on
both the number of volumes and the logical capacity of these volumes. If thin
provisioning is used, then the metadata is allocated for the entire volume when the
volume is created, and not when extents are used, so over-provisioned
environments have more metadata.
Metadata is allocated in units that are called metadata extents, which are 16 MB for
FB data and 21 cylinders for CKD data. There are 64 metadata extents in each user
extent for FB and 53 for CKD. The metadata space usage is as follows:
v Each volume takes one metadata extent.
v Ten extents (or part thereof) for the volume take one metadata extent.
For example, both a 3390-3 and a 3390-9 volume each take two metadata extents
and a 128 GB FB volume takes 14 metadata extents.
Chapter 4. Storage system physical configuration
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