Understanding Bootstrap Of Oracle Database

In this Document

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Applies to:

Oracle Server – Enterprise Edition –Version:
Information in this document applies to any platform.
Oracle RDBMS Server

Purpose

What is bootstrap?
What happens during database startup?
Which objects are classified as bootstrap objects in oracle database?
How the dictionaries are loaded?
Why bootstrap (ORA-00704) process failure is so serious if it occurs on startupor while recovery?

Scope and Application

This article intend to explain thebootstrap operations of oracle.

Understanding Bootstrap Of Oracle Database

What is bootstrap?

In general, Bootstrap is a technique forloading the first few instructions of a computer program into active memory andthen using them to bring in the rest of the program.

What is bootstrap in Oracle ?

In Oracle, Bootstrap refers to loading of metadata (data dictionary) before we OPEN the database.
Bootstrap objects are classified as the objects (tables / indexes / clusters)with the object_id below 56 as bootstrap objects.
These objects are mandatory to bring up an instance, as this contains the mostimportant metadata of the database.

What happens on database startup?

This shall be explained by setting theSQL_TRACE while opening the database.

Connect as sysdba and do the following

STARTUP MOUNT;
ALTER SESSION SET EVENTS ’10046 TRACE NAME CONTEXT FOREVER, LEVEL 12′;
ALTER DATABASE OPEN;
ALTER SESSION SET EVENTS ’10046 TRACE NAME CONTEXT OFF’;
SHOW PARAMETER USER_DUMP_DEST
ORADEBUG SETMYPID
ORADEBUG TRACEFILE_NAME

The sql_trace of the above process explainsthe following operations behind startup.
The bootstrap operation happens between MOUNT stage and OPEN stage.

1. The first SQL after in the above trace shows the creation of the bootstrap$ table.
Something similar to the following:

———————————————–
create table bootstrap$ ( line# number not null, obj# number not null, sql_textvarchar2(4000) not null) storage (initial 50K objno 56 extents (file 1 block377))
———————————————–

This sys.bootstrap$ table contains theDDL’s for other bootstrap tables (object_id below 56).

-> Actually these tables were createdinternally by the time of database creation (by sql.bsq), The create DDL passedbetween MOUNT and OPEN stage will be executed through different driverroutines. In simple words these are not standard CREATE DDLs.
While starting up the database oracle will load these objects into memory(shared_pool),(ie) it will assign the relevant object number and refer to thedatafile and the block associated with that.
And such operations happen only while warm startup.

@ The internals of the above explained in‘kqlb.c’.

2. Now a query executedagainst the sys.bootstrap$ table, which holds the create sql’s for other basetables.

select line#, sql_text from bootstrap$where obj# != :1 (56)

Subsequently it willcreate those objects by running those queries.

Object number 0 – (System Rollback Segment)
Object number 2 to 55 (Other base tables)
Object number 1 is NOT used by any of the objects.

3. Performs variousoperations to keep the bootstrap objects in consistent state.

- Upon the successful completion ofbootstrap the database will do the other tasks like recovery and will open thedatabase.

Which objects are classified as bootstrapobjects in oracle database?

Objects with data_object_id less than 56are classified as core bootstrap objects.

The objects are added tothe bootstrap. The objects affected are:

hist_head$
histgrm$
i_hh_obj#_col#
i_hh_obj#_intcol#
i_obj#_intcol#
i_h_obj#_col#
c_obj#_intcol#

From 10.1 the followingobjects have been added:
fixed_obj$
tab_stats$
ind_stats$
i_fixed_obj$_obj#
i_tab_stats$_obj#
i_ind_stats$_obj#
object_usage

These additional objectsshall be re-classified (or) ignored by following methods.
1. Opening the database in migrate mode
2. Using event 38003

Event 38003 affects the bootstrap process of loading the fixed cache in kqlblfc(). Per default certain objects are marked as bootstrap objects (eventhough they are not defined as such in sys.bootstrap$) but by setting the eventthey will be left as non-bootstrapped.

What is bootstrap process failure?ORA-00704

This ORA-00704 error SERIOUS if reported atstartup. This error refers to some problem during bootstrap operation.
Any ORA-00704 error on STARTUP / RECOVER is serious, this error normally rosedue to some inconsistency with the bootstrap segments (or) data corruption onbootstrap$ (or) any of the base tables below object_id 56. After this error itmight not allow to open that database.

When ORA-00704 shall occur?

1. There is a probable of this error whenany unsupported operations are tried to force open the database.
2. This error can also occur when system datafile has corrupted blocks.(ORA-01578)
3. In earlier releases of oracle (prior to 7.3.4 and 8.0.3) this issue shallarise due to Bug 434596

The option is to restore it from a goodbackup and recover it.
-> If the underlying cause is physical corruption that is due to hardwareproblems then do complete recovery.
-> If the issue is not relating to any physical corruption, then the problemcould be due some unsupported actions on Bootstrap, and a Point In TimeRecovery would be an option in such cas

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