Database Reference
In-Depth Information
MSSM is the legacy implementation in Oracle. It has been around for many years, over many versions. ASSM
was first introduced in Oracle9
i
Release 1 and its design intention was to eliminate the need to fine tune the myriad
parameters used to control space allocation and provide high concurrency. For example, by having the
FREELISTS
parameter set to the default of 1, you might find that your insert/update intensive segments may be suffering from
contention on free space allocation. When Oracle goes to insert a row into a table, update an index key entry, or
update a row causing the row to migrate (more on that in a moment), it may need to get a block from the list of free
blocks associated with the segment. If there is only one list, only one transaction at a time may review and modify this
list—they would have to wait for each other. Multiple
FREELISTS
and
FREELIST GROUPS
serve the purpose of increasing
concurrency in such a case, as the transactions may each be looking at different lists and not contending with each other.
When I discuss the storage settings shortly, I will mention which are for manual and which are for automatic
segment space management, but in the area of storage/segment characteristics, the only storage settings that apply to
ASSM segments are as follows:
•
BUFFER_POOL
•
PCTFREE
•
INITRANS
MAXTRANS
(only in 9
i
; in 10
g
and above this is ignored for all segments)
The remaining storage and physical attribute parameters do not apply to ASSM segments.
Segment space management is an attribute inherited from the tablespace in which a segment is contained (and
segments never span tablespaces). For a segment to use ASSM, it would have to reside in a tablespace that supported
that method of space management.
•
High-water Mark
This is a term used with table segments stored in the database. If you envision a table, for example, as a flat structure
or as a series of blocks laid one after the other in a line from left to right, the
high
-
water mark
(
HWM
) would be the
rightmost block that ever contained data, as illustrated in Figure
10-1
.
