z/VSE: 45 Years of Progress - z/VM - IBM

is concerned with minimizing locking
and contention issues and providing
current data to applications as quickly
as possible.
There are several scenarios where an
application requires high data availability;
those that call for high-speed data
update and retrieval (perhaps in a 24x7
environment), include a data archiving
or data retention process, or require
coordinated DB2 utility execution with
SQL access.
In each of these cases, there are database
design choices that may alleviate
future contention problems. The most
common design solutions involve combinations
of horizontal and vertical data
partitioning with a method of assigning
clustering keys. Such a solution helps
avoid data hot spots, which are areas in
memory and on DASD where multiple
I/Os from multiple transactions may
cause contention or locking issues.
Horizontal partitioning is the separation
of table rows into two or more
physical data sets. It may be accompanied
by a similar physical partitioning
of some indexes. DB2 partitioned tables
let the designer choose a row storage
strategy based on the partitioning.
Horizontal partitioning will affect the
database backup and recovery methods
and elapsed times. Figure 2 lists some
strategies and their usefulness.
Vertical partitioning involves splitting
a table vertically—designating some
columns to remain in the table while
others are moved to another table or
tables. In this way, a single table is
denormalized into two or more tables,
each containing a subset of columns
from the original. This is usually accompanied
by replicating the primary key of
the original table across all the resulting
tables. To re-create an original row, the
tables must be joined on their primary
key columns. The DBA usually chooses
vertical partitioning for performance
reasons; Figure 3 lists additional decision-making
factors.
Key clustering is a technique the
DBA uses to ensure inserted rows don’t
create hot spots in the data. One common
way this happens is when new
records are assigned a key based on a
sequentially ascending value (such as an
identity column or a sequence) and the
table is clustered on this key. The result
is that newly inserted rows are added at
the physical end of the table.
Assuming the DBA implements
page-level locking, a row insert requires
an exclusive lock on the page where the
row is placed, and the lock is kept until
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