Troubleshooting ORA-1555&nb…

 Troubleshooting ORA-1555 (Doc ID
1307334.1)
By khrobins on
May 16, 2011

This article is intended to assist in finding tips and
techniques to assist with finding solutions to ORA-1555 errors. The
document will cover the following topics:

Concepts/Definitions

Diagnosing

Common Causes/Solutions
Concepts/Definitions

The ORA-1555 errors can happen when a query is unable
to access enough undo to build

a copy of the data at the time the query started. Committed
“versions” of blocks are

maintained along with newer uncommitted “versions” of those blocks
so that queries can

access data as it existed in the database at the time of the query.
These are referred to as

“consistent read” blocks and are maintained using Oracle undo
management.

See
Note 40689.1 - ORA-1555 "Snapshot too old" - Detailed Explanation
for more about

these errors.
Diagnosing

Due to space limitations, it is not always feasible to
keep undo blocks on hand for the life of the instance. Oracle
Automatic Undo Management (AUM) helps to manage the time frame that
undo blocks are stored. The time frame is the “retention” time for
those blocks.

There are several ways to investigate the ORA-1555 error. In most
cases, the error is a legitimate problem with getting to an undo
block that has been overwritten due to the undo “retention” period
having passed.

AUM will automatically tune up and down the “retention” period, but
often space limitations or configuration of the undo tablespace
will throttle back continuous increases to the “retention”
period.

The error message is reported in the user session and often is not
captured in the alert log. The user could see a message like

Using rollback segment functionality:

ORA-1555: snapshot too old (rollback segment too small)

or

Using AUM:

ORA-01555: snapshot too old: rollback segment number 9 with name
"_SYSSMU9$" too small

If the error is captured in the alert.log, you would see something
like

Tue May 26 16:16:57 2009

ORA-01555 caused by SQL statement below (SQL ID: 54yn3n36w24ft,
Query Duration=922 sec, SCN: 0x0007.8a55f4e3)
Initial Investigation
Rollback Segments:

With Oracle 10g and later versions of Oracle, you can
still use a Rollback Segments configuration. ORA-1555 errors in
that environment still follow older guidelines as described
in

Note 10579.1 - How many Rollback Segments to
Have
Note
107085.1 - Tuning Rollback Segments
Note
69464.1 - Rollback Segment Configuration & Tips
Automatic Undo Management:

The database will be self tuning for undo when using
Automatic Undo Management. This does not eliminate ORA-1555
completely, but does minimize ORA-1555 as long as there is adequate
space in the undo tablespace and workloads tend to follow
repeatable patterns. In some cases with periodic changes to
workload (large data updates particularly with LOB data) the self
tuning of undo can become aggressive and lead to undo issues.

Note
461480.1 - FAQ Automatic Undo Management (AUM) / System Managed
Undo (SMU)
Note
135053.1 -How to Create a Database with Automatic Undo
Management
Note
268870.1 - How to Shrink the datafile of Undo
Tablespace
Note
231776.1 - How to switch a Database from Automatic Undo
Management (AUM) back to using Rollback
Segments
Note
296863.1 - How to Keep All UNDO Segments from Being Offlined in
Oracle 10g - Fast Ramp-Up
LOB Issues:

Out-of-row LOB undo is maintained in the LOB segment.
So the UNDO tablespace and undo retention is not associated with
most LOB ORA-1555 issues. Instead the LOB column is created using
either PCT_VERSION or RETENTION to manage how much space within
blocks or time transpires before the LOB undo is overwritten. In
environments with high updates, deletes on rows including LOBs, the
chances of ORA-1555 on LOB undo is very high.

PCT_VERSION and RETENTION are not auto-tuned. To “tune” those
configuration settings, you must change the values for PCT_VERSION
or RETENTION. Changes to UNDO_RETENTION does not change LOB
retention time frames.

Note
162345.1 - LOBS - Storage, Read-consistency and
Rollback
Note
386341.1 - How to determine the actual size of the LOB segments
and how to free the deleted/unused space above/below the
HWM
Note
563470.1 – Lob retention not changing when undo_retention is
changed
Note
422826.1 – How to identify LOB Segment Use PCTVERSION or
RETENTION from Data Dictionary
Error Tracing

Undo error tracing can be done for normal undo
operations using the following events:

NOTE: Normal undo operations will be indicated in the error message
in that the error message includes a segment name like

…. name "_SYSSMU1$" too small

If the error doesn’t show a segment name

… name "" too small

the problem is often related to LOB undo

If using pfile:

event="10442 trace name context forever, level 10"

If using spfile:

Alter system set events '10442 trace name context forever, level
10';

Reproduce the ORA-1555 error and upload the trace file to Oracle
Support.

LOB undo error tracing is more difficult. Set additional tracing
events as follows:

Start Session 1

Alter session set events '10046 trace name context forever, level
12';

Reproduce the error

Exit Session 1

Start Session 2

Alter session set events '10051 trace name context forever, level
1';

Reproduce the error

Exit Session 2

Start Session

Alter session set events '1555 trace name errorstack forever, level
3';

Reproduce the error

Exit Session 3

Additional resources to review:
Note
846079.1 – LOBs and ORA-1555
troubleshooting
Note
253131.1 –Concurrent Writes May Corrupt LOB Segment When Using
Auto Segment Space Management
Note
467872.1 – TROUBLESHOOTING GUIDE (TSG) – ORA-1555
V$UNDOSTAT Analysis

The V$UNDOSTAT view holds undo statistics for 10
minute intervals. This view

represents statistics across instances, thus each begin time, end
time, and

statistics value will be a unique interval per instance.

This does not track undo related to LOB
Note
262066.1 – How To Size UNDO Tablespace For Automatic Undo
Management
Note
1112363.1 – When Does Undo Used Space Become
Available?
Note
240746.1 – 10g NEW FEATURE on AUTOMATIC UNDO
RETENTION
Diagnostics Scripts

Refer to
Note 746173.1 : Common Diagnostic Scripts for AUM problems

and
Note 877613.1 : AUM Common Analysis/Diagnostic Scripts
Common Causes/Solutions
Using Rollback Segments functionality:

* Problem happening on SYSTEM tablespace that still
uses old Rollback Segment functionality even when configured for
Automatic Undo Management (AUM).

* There are not enough rollback segments to manage the undo needed
for long running queries.

* Rollback Segments are too small and undo is overwritten before
long running queries complete.
Reference:

Note 69464.1 – Rollback Segment Configuration &
Tips
Note
10630.1 – ORA-1555: “Snapshot too old” –
Overview
Note
862469.1 – ORA-604 & ORA-1555 Rollback Segment 0 with Name
“System” Too Small
Using Automatic Undo Management (AUM):

* TUNED_UNDORETENTION in V$UNDOSTAT around the time of
the error is lower than the QUERY DURATION indicated in the error
message. This is a legitimate ORA-1555 and if queries are going to
run for very long time frames, UNDO_RETENTION may need to be
larger. Auto-tuned retention may not be able to keep up with the
undo workload and staying within space limitations on the UNDO
tablespace.

* LOB updates and/or deletes are frequent and a higher PCT_VERSION
is required to provide enough space in the LOB Segment to
accommodate the LOB undo. RETENTION on LOBs that are updated or
deleted frequently can run into problems holding UNDO long enough
for queries.

* QUERY DURATION shown in the error message is 30+ years and
therefore, no amount of undo will satisfy the consistent read
blocks.

Note
750195.1 – ORA-1555 Shows Unrealistic Query Duration (billions
of seconds)

* QUERY DURATION shown in the error message is 0. NOTE: This has
been filed as a bug on many release levels and has been very
difficult to narrow down to a specific problem.

Note
761128.1 – ORA-1555 Error when Query Duration as 0
Seconds

* QUERY DURATION is lower than TUNED_UNDRETENTION. Undo header
information can sometimes get overwritten or you could be seeing a
bug.

* TUNED_UNDORETENTION stays very high and UNDO tablepsace continues
to grow continuously or getting space errors.

Note
1112431.1 – Undo Remains Unexpired When Using Non-autoextensible
Datafiles for Undo Tablespace.
Additional Reference:

Database Administration Community