Chapter 1. Introduction to the Parallel Sysplex 23
Figure 1-12 Managing IRLM failures
Lock structure sizing considerations
Locking services will work no matter how big the lock table is, but if there are a lot of false
contentions, then there will be a lot of additional overhead to resolve the potential conflict.
Ideally we would like to have no potential contentions, but since this is not really possible, we
would at least like it to be a very, very small probability. Therefore, the size of the structure
and the size of the lock table should be big enough to keep this probability low. A lock table
entry may require two, four, or eight bytes, depending on how many IRLMs may be connected
to it. A two-byte entry can handle up to six IRLMs. Assuming there are no more than six
IRLMs in your data sharing group, a 32 MB structure with a 16 MB lock table will have 8
million entries. If there are seven to 23 IRLMs, then the entry size is four bytes and the 16 MB
lock table will contain only four million entries. And 24 to 32 IRLMs will require 8-byte entries,
producing only 2 million entries. The first connector sets the size of the lock table and the
maximum number of connectors. It is important that the IRLM sets this at the lowest possible
value.
Help with sizing the IRLM lock structure can be found at:
http://www.ibm.com/servers/eserver/zseries/cfsizer
1.6.5 List structures and list services
List services allow subsystems and authorized applications running in the same Parallel
Sysplex to use a Coupling Facility to share data organized in a list structure. The list structure
consists of a set of lists and an optional lock table. Information is stored on each list as a
series of list entries. The list entries on one list may be kept in sorted order by key value. The
lock table can be used to serialize on resources in the list structure, such as a particular list or
list entry. A list structure with a lock table is called a serialized list structure.
XCF status monitoring services
informs other members of group
Group user routine driven
All IRLMs are part of same
XCF group
Its update locks in record list remain
in lock structure as
retained locks
Partner lock managers read retained locks
Save in data space
Any lock request for retained lock
is immediately rejected
No need to query lock structure
Lock reject condition (U3303)
I
R
L
M
Lock
Services
Locks
XCF
Lock Structure
I
R
L
M
Lock
Services
Locks
24 IMS in the Parallel Sysplex, Volume I: Reviewing the IMSplex Technology
While cache and lock services have very distinct purposes, the connector to a list structure
may use it to keep
any kind of information that needs to be shared. For example, in an
IMSplex, list structures and list services are used for three functions:
򐂰 IMS uses the
resource list structure for resource management. This structure is used to
keep
status information about the resources in the IMSplex, such as which nodes are
logged on to which IMS, which users are in a conversation and what the conversation ID
is, what the names are of the online change data sets, and other information related to the
IMSplex.
򐂰 CQS uses up to four
shared message queue list structures for shared message queues.
These structures are used to keep
messages (IMS full function and Fast Path EMH
transactions and responses) that need to be shared among all the IMSs in a shared
queues (XCF) group.
򐂰 CQS uses the System Logger to log activity to the shared queues structures. The System
Logger uses one or two (full function and Fast Path)
logger list structures to support a
shared logstream among all the CQSs in the shared queues group.
Other users of list structures include XCF for signalling, JES2 checkpoint, VTAM Generic
Resources and multinode persistent sessions, and just about anything that needs to be
shared across the sysplex and which doesnt meet the specific requirements of cache or lock
services.
Components of a list structure
The list structure has more components that either cache or lock structures. Figure 1-13
shows the major components of a list structure. Some of these components exist in every list
structure, and some are optional. The first connector to a list structure determines its
attributes and whether or not the structure will have these optional components.

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