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ASSM argh!

Here’s a problem with ASSM that used to exist in older versions of Oracle had disappeared by 11.2.0.4 and then re-appeared in 12.1.0.2 – disappearing again by 12.2.0.1. It showed up on MoS a few days ago under the heading: “Insert is running long with more waits on db file sequential read”.

The obvious response to this heading is to question the number of indexes on the table – because big tables with lots of indexes tend to give you lots of random I/O as Oracle maintains the indexes – but this table had no indexes. The owner of the problem supplied several of bits of information in the initial post, with further material in response to follow-up questions, including the tkprof summary of the 10046/level 12 trace of the insert and two extracts from the trace file to show us some of the “db file sequential read” waits – the first extract made me wonder if there might be some issue involving 16KB blocks but the second one dispelled that illusion.

There are several buggy things that can appear with ASSM and large-scale DML operations, and sometimes the problems can appear long after the original had done the dirty deed, so I thought I’d create a simple model based on the information supplied to date – and discovered what the problem (probably) was. Here’s how it starts – I’ve created a tablespace using ASSM, and in this tablespace I’ve created a table which has 48 columns with a row length of 290 bytes (roughly matching the OP’s table), and I’ve hacked out a simple PL/SQL block that loops around inserting arrays of 100 rows at a time into the table for a total of 1M rows before committing.


rem
rem     Script:         assm_cleanout.sql
rem     Author:         Jonathan Lewis
rem     Dated:          Jan 2018
rem     Purpose:
rem
rem     Last tested
rem             12.2.0.1
rem             12.1.0.2        Lots of blocks left "not full"
rem             11.2.0.4
rem

rem
rem     using OMF, so no file-name needed
rem     Ran this bit as SYS, and altered test user to have unlimited quota
rem

/*
create
        tablespace test_8k_assm
        datafile size 1G
        extent management local
        autoallocate
        segment space management auto
;
*/

rem
rem     Create the table, pre-allocate some space.
rem     This means we should get consistent 8M extents and not initial little ones
rem

create table t1 (
        v001 varchar2(5), v002 varchar2(5), v003 varchar2(5), v004 varchar2(5), v005 varchar2(5),
        v006 varchar2(5), v007 varchar2(5), v008 varchar2(5), v009 varchar2(5), v010 varchar2(5),
        v011 varchar2(5), v012 varchar2(5), v013 varchar2(5), v014 varchar2(5), v015 varchar2(5),
        v016 varchar2(5), v017 varchar2(5), v018 varchar2(5), v019 varchar2(5), v020 varchar2(5),
        v021 varchar2(5), v022 varchar2(5), v023 varchar2(5), v024 varchar2(5), v025 varchar2(5),
        v026 varchar2(5), v027 varchar2(5), v028 varchar2(5), v029 varchar2(5), v030 varchar2(5),
        v031 varchar2(5), v032 varchar2(5), v033 varchar2(5), v034 varchar2(5), v035 varchar2(5),
        v036 varchar2(5), v037 varchar2(5), v038 varchar2(5), v039 varchar2(5), v040 varchar2(5),
        v041 varchar2(5), v042 varchar2(5), v043 varchar2(5), v044 varchar2(5), v045 varchar2(5),
        v046 varchar2(5), v047 varchar2(5), v048 varchar2(5)
)
segment creation immediate
tablespace test_8k_assm
storage(initial 8M)
;

alter table t1 allocate extent (size 8M);
alter table t1 allocate extent (size 8M);

rem
rem     Simple anonymous pl/sql block
rem     Large insert, handled with array inserts
rem     Can modify loop count and array size very easily
rem

declare
        type tab_array is table of t1%rowtype;
        junk_array tab_array;
begin

        select
                'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx',
                'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx',
                'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx',
                'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx',
                'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx', 'xxxxx'
        bulk collect into
                junk_array
        from
                all_objects
        where
                rownum  <= 100 -- > comment to avoid WordPress format issue
        ;

        for i in 1..10000 loop
                forall j in 1..junk_array.count
                        insert into t1 values junk_array(j) ;
        end loop;

end;

commit;

The number of rows per block after this insert is 24, with 1038 bytes free space left (808 due to the pctfree = 10, then the bit that was too small to take a 25th row before breaching the pctfree barrier). This means we should report 1M/24 = 41,666 full blocks and one block with some free space. So we query the table using the dbms_space package:


declare
        m_unformatted_blocks    number;
        m_unformatted_bytes     number;
        m_fs1_blocks            number;
        m_fs1_bytes             number;
        m_fs2_blocks            number;
        m_fs2_bytes             number;

        m_fs3_blocks            number;
        m_fs3_bytes             number;
        m_fs4_blocks            number;
        m_fs4_bytes             number;
        m_full_blocks           number;
        m_full_bytes            number;

begin
        dbms_space.SPACE_USAGE(
                segment_owner           => 'TEST_USER',
                segment_name            => 'T1',
                segment_type            => 'TABLE',
                unformatted_blocks      => m_unformatted_blocks,
                unformatted_bytes       => m_unformatted_bytes,
                fs1_blocks              => m_fs1_blocks ,
                fs1_bytes               => m_fs1_bytes,
                fs2_blocks              => m_fs2_blocks,
                fs2_bytes               => m_fs2_bytes,
                fs3_blocks              => m_fs3_blocks,
                fs3_bytes               => m_fs3_bytes,
                fs4_blocks              => m_fs4_blocks,
                fs4_bytes               => m_fs4_bytes,
                full_blocks             => m_full_blocks,
                full_bytes              => m_full_bytes
        );

        dbms_output.new_line;
        dbms_output.put_line('Unformatted                   : ' || to_char(m_unformatted_blocks,'999,999,990') || ' / ' || to_char(m_unformatted_bytes,'999,999,999,990'));
        dbms_output.put_line('Freespace 1 (  0 -  25% free) : ' || to_char(m_fs1_blocks,'999,999,990') || ' / ' || to_char(m_fs1_bytes,'999,999,999,990'));
        dbms_output.put_line('Freespace 2 ( 25 -  50% free) : ' || to_char(m_fs2_blocks,'999,999,990') || ' / ' || to_char(m_fs2_bytes,'999,999,999,990'));
        dbms_output.put_line('Freespace 3 ( 50 -  75% free) : ' || to_char(m_fs3_blocks,'999,999,990') || ' / ' || to_char(m_fs3_bytes,'999,999,999,990'));
        dbms_output.put_line('Freespace 4 ( 75 - 100% free) : ' || to_char(m_fs4_blocks,'999,999,990') || ' / ' || to_char(m_fs4_bytes,'999,999,999,990'));
        dbms_output.put_line('Full                          : ' || to_char(m_full_blocks,'999,999,990') || ' / ' || to_char(m_full_bytes,'999,999,999,990'));

end;
/

The results aren’t what we expect:


Unformatted                   :            0 /                0
Freespace 1 (  0 -  25% free) :       35,001 /      286,728,192
Freespace 2 ( 25 -  50% free) :            1 /            8,192
Freespace 3 ( 50 -  75% free) :            0 /                0
Freespace 4 ( 75 - 100% free) :           67 /          548,864
Full                          :        6,665 /       54,599,680

We have one block marked as 25 – 50% free (that’s the one block with 16 rows in it, which means about 40% space currently free) but our 41,666 full blocks are actually reported as 6,665 full blocks and 35,001 blocks with some space available. That’s going to hurt eventually if some process wants to insert more rows and finds that it has to fail its way through 35,001 blocks before finding a block which has enough free space.

So what happens when I repeat the PL/SQL block (and commit)? Here are the results from calls to dbms_space after the next two cycles:


Unformatted                   :            0 /                0
Freespace 1 (  0 -  25% free) :       70,002 /      573,456,384
Freespace 2 ( 25 -  50% free) :            2 /           16,384
Freespace 3 ( 50 -  75% free) :            0 /                0
Freespace 4 ( 75 - 100% free) :          220 /        1,802,240
Full                          :       13,330 /      109,199,360

Unformatted                   :          256 /        2,097,152
Freespace 1 (  0 -  25% free) :      105,003 /      860,184,576
Freespace 2 ( 25 -  50% free) :            3 /           24,576
Freespace 3 ( 50 -  75% free) :            0 /                0
Freespace 4 ( 75 - 100% free) :          117 /          958,464
Full                          :       19,995 /      163,799,040

Every time we execute the PL/SQL block we leave a trail of 35,001 more blocks which are flagged as “not quite full”.

Looking at the session stats while running the insert loop I can tell that Oracle isn’t checking to see whether or not it should be using those blocks. (A quick way of proving this is to flush the buffer cache before each execution of the PL/SQL and note that Oracle doesn’t read back the 105,000 blocks before inserting any data). So somehow, sometime, someone might get a nasty surprise – and here’s one way that it might happen:

Since I know I my data fits 24 rows per block I’m going to modify my PL/SQL block to select one row into the array then loop round the insert 25 times – so I know I’m inserting a little bit more than one block’s worth of data. Starting from the state with 105,003 blocks marked as “Freespace 1” this is what I saw – first, the free space report after inserting 25 rows:


Unformatted                   :          240 /        1,966,080
Freespace 1 (  0 -  25% free) :        1,074 /        8,798,208
Freespace 2 ( 25 -  50% free) :            0 /                0
Freespace 3 ( 50 -  75% free) :            0 /                0
Freespace 4 ( 75 - 100% free) :          133 /        1,089,536
Full                          :      123,927 /    1,015,209,984

Then a few wait events and session statistics for the insert:


---------------------------------------------------------
SID:    39:TEST_USER - jonathan
Session Events - 09-Jan 16:57:18
Interval:-      6 seconds
---------------------------------------------------------
Event                                             Waits   Time_outs        Csec    Avg Csec    Max Csec
-----                                             -----   ---------        ----    --------    --------
db file sequential read                          15,308           0         128        .008           3
db file scattered read                           20,086           0         271        .014           4

---------------------------------
Session stats - 09-Jan 16:57:18
Interval:-  6 seconds
---------------------------------
Name                                                                     Value
----                                                                     -----
session logical reads                                                  269,537
physical read total IO requests                                         35,401
db block gets                                                          229,522
consistent gets                                                         40,015
physical reads                                                         124,687
physical reads cache                                                   124,687
db block changes                                                       208,489
physical reads cache prefetch                                           89,293
redo entries                                                           207,892
redo size                                                           16,262,724
undo change vector size                                                  1,720
deferred (CURRENT) block cleanout applications                         103,932
table scan blocks gotten                                                20,797
HSC Heap Segment Block Changes                                              25

The session has read and updated almost all of the level 1 bitmap blocks. I don’t know exactly what triggered this re-read, but seems to be related to the number of rows inserted (or, perhaps, the amount of space used rather than the row count) as an insert crosses the pctfree boundary and fails over to the next block. I’ve only done a couple of little tests to try and get a better idea of why an insert sometimes sweeps through the bitmap blocks – so I know that inserting 2 or 3 rows at a time will also trigger the cleanout – but there are probably several little details involved that need to be identified.

You might note a couple of details in the stats:

  • Because I had flushed the buffer cache before the insert Oracle did its “cache warmup” tablescanning trick – if this had not happened I would probably have done a single block read for every single bitmap block I touched.
  • There are 103,932 block cleanout applications – but 208,000 db block changes and redo entries. Roughly half the latter are for data block cleanouts (OP code 4.1) and half are the state changes on the level 1 bitmap blocks (OP code 13.22). You’ll notice that neither change produces any undo.
  • I’ve also included the HSC Heap Segment Block Changes statistics to show you that not all changes to Heap Segment Blocks show up where you might expect them.

And finally:

If you re-run the tests on 11.2.0.4 and 12.2.0.1 you get the following results after the intial script run – the problem doesn’t exist:


11.2.0.4
========
Unformatted                   :            0 /                0
Freespace 1 (  0 -  25% free) :            0 /                0
Freespace 2 ( 25 -  50% free) :            1 /            8,192
Freespace 3 ( 50 -  75% free) :            0 /                0
Freespace 4 ( 75 - 100% free) :           67 /          548,864
Full                          :       41,666 /      341,327,872

12.2.0.1
========
Unformatted                   :            0 /                0
Freespace 1 (  0 -  25% free) :            0 /                0
Freespace 2 ( 25 -  50% free) :            1 /            8,192
Freespace 3 ( 50 -  75% free) :            0 /                0
Freespace 4 ( 75 - 100% free) :           67 /          548,864
Full                          :       41,666 /      341,327,872

So how does this help the OP.

  • First, there may be a huge mess still waiting to be cleaned in the table – but at 34M blocks I’m not too keen to suggest running the dbms_space routine to find out what it looks like – but maybe that’s necessary.
  • Secondly – an upgrade to 12.2 will probably avoid the problem in future.
  • Thirdly – if the number of rows per block is very close to uniform, write a little code to do a loop that inserts (say) 2 * expected number of rows per block as single row inserts and rolls back; the inserts will probably trigger a massive though perhaps not complete cleanout, so rinse and repeat until the cleanout is complete. Try to find a time when you don’t mind the extra load to get this job done.
  • Finally – on the big job that does the bulk insert – repeat the dummy insert/rollback at the end of the job to clean up the mess made by the job.

Addenda

Prompted by comment #2 below, I should add that if the problem has been fixed in 12.2 then possibly there’s a bug report and patch for it already. If there isn’t then the OP could raise an SR (referencing this blog note), and request a bug fix or back-port from 12.2.

And with 24 hours of publication, comment #4 (from Yury Pudovchenko) tells us that the bug is fixed by the Oct 2017 Bundle Patch.