IBM TotalStorage DS6000 Series: Performance Monitoring and Tuning

Chapter 3. Logical configuration planning 63

3.5.2 Mixing open and zSeries logical disks

Open systems data is fixed block (FB) and zSeries data is count-key-data (CKD) and they are

physically different representations within the DS6000. The two data types are not able to be

mixed within an Array.

CKD

In count-key-data (CKD) organization, the data field stores the user data. Also, because the

data records can be variable in length, they all have an associated

count field that indicates

the user data record size. Then the

key field is used to enable a hardware search based on a

key. However, this is not generally used for most data anymore. Extended count-key-data

(ECKD™) is a more recent version of CKD that uses an enhanced S/390® channel command

set.

Fixed Block (FB)

Open systems data and zSeries data access patterns also tend to be different, with z-Series

data typically being stored and transferred in logical blocks that approximate a 3390-1 track

size of 56 KB, while open systems data is typically managed in 4 KB logical blocks.

3.5.3 Arrays and Array Sites

An Array in a DS6000 consists one or two Array Sites, and is analogous to a Disk Group in an

ESS (which is the logical representation of two pairs of 4 disks taken from two different

physical Ranks on the same SSA loop in an ESS). The minimum orderable base DS6000

must contain at least two disk drive sets, where a disk drive set contains four identical disk

drives (same capacity and revolutions per minute (RPM)). The minimum additional purchase

unit is one disk drive set or four DDMs.

Array Sites

The DS6000 is available with a configuration entity or Array Site consisting of one disk drive

set. A fully populated server enclosure or storage enclosure has two pairs of Array Sites, as

can be seen in Figure 3-3 on page 64 showing Array Site locations for a DS6000 server

enclosure. The DDMs selected for an Array Site will be selected from the same disk

enclosure string by the DS6000 and you have no way to influence this process. An example of

the relationship between DDMs and their associated Array Site is shown in Example 3-1 on

page 65. The Array Sites have been shown here as S1 through S4. An Array Site is the basic

building block for Array creation.

Important: The DS6000 is restricted to two address groups, and each of these can be

associated with either open systems data or with zSeries data, but not both. Refer to

“Address groups” on page 71 for further discussion.