Waits, Enqueues, and Latches - Expert Oracle RAC Performance Diagnostics and Tuning - page 588

Large objects with several concurrent transactions and with a high DML operation are candidates for low ITLs.
Normal practices followed by administrators to help reduce ITL related latch or contention is to increase the INITRANS
parameter at the object level. INITRANS is set by default to 1 for the table and 2 for indexes. For very large objects, this
should be increased after checking the V$SEGMENT_STATISTICS view for segments with high “ITL waits.” Almost always
a small increase in the INITRANS should help resolve this issue. On the contrary, increasing this to very large numbers
could also have impacts in other areas of performance.

Data BLOCK FOrMat

Figure 17-1 illustrates the data block format dissection. the data block consists of two major sections: the
header and the body or data layer. the header contains the cache layer, a fixed transaction layer, and a variable
transaction layer. the body consists of various sections of the physical data and free space. the variable
transaction layer is of importance in our discussion here. the variable transaction layer consists of 24 bytes and
represents one itL.

20 bytes

Cache layer

Block
Header

48 bytes

Transaction Layer Fixed

#ITL - 1 * 24 bytes

Transaction Layer Variable

20 bytes

Table Directory

#tables* 4 bytes
+ #rows* 2 bytes

Row Directory

Space
used
by each
layer

Free Space

Data
Layer

Row Data

4 bytes Tail

Figure 17-1. Data block structure

the number of itLs allocated is determined by the parameter INITRANS . this happens during a
Create/aLter taBLe/CLuster/indeX ddL operations. if no value is specified, Oracle allocates a default value
of 1 for tables and 2 for indexes. after the initial allocation, subsequent changes will take effect only on new
data blocks. Whereas INITRANS limits the initial number of transactions, the maximum number of transactions
is preset by Oracle to 255 (in earlier releases, the MAXTRANS parameter helped set this value; the MAXTRANS
parameter is now depreciated).

increasing the value of INITRANS would mean the space would be moved from the data block free space into the
header section, leaving less room to contain the physical data.