Oracle 9i and Undo Segments Startup No#
Did a search prior to posting, ran across some older threads I didn't think needed resurfacing, so I'm posting a new one.
I'm interested in discussing the way in which Oracle allocates initial and additional Undo segments. I've found on Metalink the following.
Note: 135090.1 Managing Rollback/Undo Segments
1. When creating an UNDO tablespace, these are automatically created:
* n undo segments (based on SESSIONS parameter value)
* named as _SYSSMUn$
* owned by PUBLIC (usable for OPS configuration)
* not manually manageable
In the Oracle 9.2 documentation:
When the instance starts up, it brings online a number of rollback segments as determined by either of the following:
The ROLLBACK_SEGMENTS initialization parameter
The TRANSACTIONS and TRANSACTIONS_PER_ROLLBACK_SEGMENT initialization parameters (Anyway, this only releates to PUBLIC rollabck segments anyway)
I have the parameters
SESSIONS = 170
TRANSACTIONS = 187
TRANSACTIONS_PER_ROLLBACK_SEGMENT = 5
And yet 10 Rollback segments were onlined *scratches head*. I'm not sure how 9.2.0. arrived at on-lining 10 segments.
Undo Segment 1 Onlined
Undo Segment 2 Onlined
Undo Segment 3 Onlined
Undo Segment 4 Onlined
Undo Segment 5 Onlined
Undo Segment 6 Onlined
Undo Segment 7 Onlined
Undo Segment 8 Onlined
Undo Segment 9 Onlined
Undo Segment 10 Onlined
Also, I found the followig information for when additional UNDO segments are onlined (Metalink: Rowena Serna)
"It will online them as needed in this matter:
* Any new txn created will attempt to bind to an undo segment with NO current active txns
* When all online undo segments have active txns, then offlined undos will be onlined
* If all offlined undo segs have been onlined, then more undo segs
will be dynamically created.
* When no more undo segs can be created (not enough space in the
tablespace) then existing segments will allow multiple active
Any comments about the onlining of segments welcomed...
OCP 8i, 9i DBA
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