Martins Blog

Trying to explain complex things in simple terms

Understanding enhancements to block cleanouts in Exadata part 2

Posted by Martin Bach on February 25, 2015

In part 1 of the series I tried to explain (probably a bit too verbose when it came to session statistics) what the effect is of delayed block cleanout and buffered I/O. In the final example the “dirty” blocks on disk have been cleaned out in the buffer cache, greatly reducing the amount of work to be done when reading them.

Catching up with now, and direct path reads. You probably noticed that the migration to 11.2 caused your I/O patterns to change. Suitably large segments are now read using direct path read not only during parallel query but also potentially during the serial execution of a query. Since the blocks read during a direct path read do not end up in the buffer cache there is an interesting side effect to block cleanouts. The scenario is the same unrealistic yet reproducible one as the one presented in part 1 of this article.

Enter Direct Path Reads – non Exadata

To be absolutely sure I am getting results without any optimisation offered by Exadata I am running the example on different hardware. I am also using 11.2.0.4 because that’s the version I had on my lab server. The principles here apply between versions unless I am very mistaken.

I am repeating my test under realistic conditions, leaving _serial_direct_read at its default, “auto”. This means that I am almost certainly going to see direct path reads now when scanning my table. As Christian Antognini has pointed out direct path reads have an impact on the amount of work that has to be done. The test commences with an update in session 1 updating the whole table and flushing the blocks in the buffer cache to disk to ensure they have an active transaction in the ITL part of the header. The selects on session 2 show the following behaviour. The first execution takes longer as expected, the second one is faster.

SQL> select /* not_exadata */ count(*) from t1_100k;

  COUNT(*)
----------
    500000

Elapsed: 00:00:20.62

SQL> @scripts/mystats stop r=physical|cleanout|consistent|cell|table
...
------------------------------------------------------------------------------------------
2. Statistics Report
------------------------------------------------------------------------------------------

Type    Statistic Name                                                               Value
------  ----------------------------------------------------------------  ----------------
STAT    active txn count during cleanout                                            83,334
STAT    cell physical IO interconnect bytes                                    747,798,528
STAT    cleanout - number of ktugct calls                                           83,334
STAT    cleanouts and rollbacks - consistent read gets                              83,334
STAT    consistent changes                                                           1,019
STAT    consistent gets                                                            751,365
STAT    consistent gets - examination                                              667,178
STAT    consistent gets direct                                                      83,334
STAT    consistent gets from cache                                                 668,031
STAT    consistent gets from cache (fastpath)                                          850
STAT    data blocks consistent reads - undo records applied                        583,334
STAT    immediate (CR) block cleanout applications                                  83,334
STAT    physical read IO requests                                                    8,631
STAT    physical read bytes                                                    747,798,528
STAT    physical read total IO requests                                              8,631
STAT    physical read total bytes                                              747,798,528
STAT    physical read total multi block requests                                       673
STAT    physical reads                                                              91,284
STAT    physical reads cache                                                         7,950
STAT    physical reads direct                                                       83,334
STAT    table fetch by rowid                                                           170
STAT    table scan blocks gotten                                                    83,334
STAT    table scan rows gotten                                                     500,000
STAT    table scans (direct read)                                                        1
STAT    table scans (short tables)                                                       1


------------------------------------------------------------------------------------------
3. About
------------------------------------------------------------------------------------------
- MyStats v2.01 by Adrian Billington (http://www.oracle-developer.net)
- Based on the SNAP_MY_STATS utility by Jonathan Lewis

==========================================================================================
End of report
==========================================================================================

The second execution shows slightly better performance as seen before in part 1. The relevant statistics are shown here:


SQL> select /* not_exadata */ count(*) from t1_100k;

  COUNT(*)
----------
    500000

Elapsed: 00:00:04.60

SQL> @scripts/mystats stop r=physical|cleanout|consistent|cell|table
...
------------------------------------------------------------------------------------------
2. Statistics Report
------------------------------------------------------------------------------------------

Type    Statistic Name                                                               Value
------  ----------------------------------------------------------------  ----------------
STAT    active txn count during cleanout                                            83,334
STAT    cell physical IO interconnect bytes                                    682,672,128
STAT    cleanout - number of ktugct calls                                           83,334
STAT    cleanouts and rollbacks - consistent read gets                              83,334
STAT    consistent changes                                                             472
STAT    consistent gets                                                            750,250
STAT    consistent gets - examination                                              666,671
STAT    consistent gets direct                                                      83,334
STAT    consistent gets from cache                                                 666,916
STAT    consistent gets from cache (fastpath)                                          245
STAT    data blocks consistent reads - undo records applied                        583,334
STAT    immediate (CR) block cleanout applications                                  83,334
STAT    physical read IO requests                                                      681
STAT    physical read bytes                                                    682,672,128
STAT    physical read total IO requests                                                681
STAT    physical read total bytes                                              682,672,128
STAT    physical read total multi block requests                                       673
STAT    physical reads                                                              83,334
STAT    physical reads direct                                                       83,334
STAT    table fetch by rowid                                                             1
STAT    table scan blocks gotten                                                    83,334
STAT    table scan rows gotten                                                     500,000
STAT    table scans (direct read)                                                        1


------------------------------------------------------------------------------------------
3. About
------------------------------------------------------------------------------------------
- MyStats v2.01 by Adrian Billington (http://www.oracle-developer.net)
- Based on the SNAP_MY_STATS utility by Jonathan Lewis

==========================================================================================
End of report
==========================================================================================

The amount of IO is slightly less. Still all the CR processing is done for every block in the table-each has an active transaction (active txn count during cleanout). As you would expect the buffer cache has no blocks from the segment stored:

SQL> select count(*), inst_id, status from gv$bh 
  2  where objd = (select data_object_id from dba_objects where object_name = 'T1_100K') 
  3  group by inst_id, status;

  COUNT(*)    INST_ID STATUS
---------- ---------- ----------
     86284          2 free
    584983          1 free
         1          1 scur

Subsequent executions all take approximately the same amount of time. Every execution has to perform cleanouts and rollbacks-no difference to before really except that direct path reads are used to read the table. No difference here to the buffered reads, with the exception that there aren’t any blocks from T1_100K in the buffer cache.

Commit

Committing in session 1 does not have much of an effect-the blocks read by the direct path read are going to the PGA instead of the buffer cache. A quick demonstration: the same select has been executed after the transaction in session 1 has committed.

SQL> select /* not_exadata */ count(*) from t1_100k;

  COUNT(*)
----------
    500000

Elapsed: 00:00:04.87

SQL> @scripts/mystats stop r=physical|cleanout|consistent|cell|table
...
------------------------------------------------------------------------------------------
2. Statistics Report
------------------------------------------------------------------------------------------

Type    Statistic Name                                                               Value
------  ----------------------------------------------------------------  ----------------
STAT    cell physical IO interconnect bytes                                    682,672,128
STAT    cleanout - number of ktugct calls                                           83,334
STAT    cleanouts only - consistent read gets                                       83,334
STAT    commit txn count during cleanout                                            83,334
STAT    consistent changes                                                             481
STAT    consistent gets                                                            166,916
STAT    consistent gets - examination                                               83,337
STAT    consistent gets direct                                                      83,334
STAT    consistent gets from cache                                                  83,582
STAT    consistent gets from cache (fastpath)                                          245
STAT    immediate (CR) block cleanout applications                                  83,334
STAT    physical read IO requests                                                      681
STAT    physical read bytes                                                    682,672,128
STAT    physical read total IO requests                                                681
STAT    physical read total bytes                                              682,672,128
STAT    physical read total multi block requests                                       673
STAT    physical reads                                                              83,334
STAT    physical reads direct                                                       83,334
STAT    table fetch by rowid                                                             1
STAT    table scan blocks gotten                                                    83,334
STAT    table scan rows gotten                                                     500,000
STAT    table scans (direct read)                                                        1


------------------------------------------------------------------------------------------
3. About
------------------------------------------------------------------------------------------
- MyStats v2.01 by Adrian Billington (http://www.oracle-developer.net)
- Based on the SNAP_MY_STATS utility by Jonathan Lewis

==========================================================================================
End of report
==========================================================================================

As you can see the work done for the cleanout is repeated very time, despite the fact that active txn count during cleanout is not found in the output. And still, there aren’t really any blocks pertaining to T1_100K in the buffer cache.

SQL> select count(*), inst_id, status from gv$bh where
  2   objd = (select data_object_id from dba_objects where object_name = 'T1_100K')
  3  group by inst_id, status;

  COUNT(*)    INST_ID STATUS
---------- ---------- ----------
     86284          2 free
    584983          1 free
         1          1 scur

OK that should be enough for now-over to the Exadata.

Direct Path Reads – Exadata

The same test again, but this time on an X2-2 quarter rack running database 12.1.0.2 and cellsrv 12.1.2.1.0. After the update of the entire table and the flushing of blocks to disk, here are the statistics for the first and second execution.

SQL> select count(*) from t1_100k;

  COUNT(*)
----------
    500000

Elapsed: 00:00:17.41

SQL> @scripts/mystats stop r=physical|cleanout|consistent|cell|table


------------------------------------------------------------------------------------------
2. Statistics Report
------------------------------------------------------------------------------------------

Type    Statistic Name                                                               Value
------  ----------------------------------------------------------------  ----------------
STAT    active txn count during cleanout                                            83,334
STAT    cell IO uncompressed bytes                                             682,672,128
STAT    cell blocks processed by cache layer                                        83,933
STAT    cell commit cache queries                                                   83,933
STAT    cell flash cache read hits                                                  14,011
STAT    cell num smartio automem buffer allocation attempts                              1
STAT    cell physical IO bytes eligible for predicate offload                  682,672,128
STAT    cell physical IO interconnect bytes                                    804,842,256
STAT    cell physical IO interconnect bytes returned by smart scan             682,904,336
STAT    cell scans                                                                       1
STAT    cleanout - number of ktugct calls                                           83,334
STAT    cleanouts and rollbacks - consistent read gets                              83,334
STAT    consistent changes                                                             796
STAT    consistent gets                                                            917,051
STAT    consistent gets direct                                                      83,334
STAT    consistent gets examination                                                833,339
STAT    consistent gets examination (fastpath)                                      83,336
STAT    consistent gets from cache                                                 833,717
STAT    consistent gets pin                                                            378
STAT    consistent gets pin (fastpath)                                                 377
STAT    data blocks consistent reads - undo records applied                        750,002
STAT    immediate (CR) block cleanout applications                                  83,334
STAT    physical read IO requests                                                   16,147
STAT    physical read bytes                                                    804,610,048
STAT    physical read requests optimized                                            14,011
STAT    physical read total IO requests                                             16,147
STAT    physical read total bytes                                              804,610,048
STAT    physical read total bytes optimized                                    114,778,112
STAT    physical read total multi block requests                                       851
STAT    physical reads                                                              98,219
STAT    physical reads cache                                                        14,885
STAT    physical reads direct                                                       83,334
STAT    table fetch by rowid                                                             1
STAT    table scan blocks gotten                                                    83,334
STAT    table scan disk non-IMC rows gotten                                        500,000
STAT    table scan rows gotten                                                     500,000
STAT    table scans (direct read)                                                        1


------------------------------------------------------------------------------------------
3. About
------------------------------------------------------------------------------------------
- MyStats v2.01 by Adrian Billington (http://www.oracle-developer.net)
- Based on the SNAP_MY_STATS utility by Jonathan Lewis

==========================================================================================
End of report
==========================================================================================


SQL> -- second execution, still no commit in session 1
SQL> select count(*) from t1_100k;

  COUNT(*)
----------
    500000

Elapsed: 00:00:01.31

SQL> @scripts/mystats stop r=physical|cleanout|consistent|cell|table
...
------------------------------------------------------------------------------------------
2. Statistics Report
------------------------------------------------------------------------------------------

Type    Statistic Name                                                               Value
------  ----------------------------------------------------------------  ----------------
STAT    active txn count during cleanout                                            83,334
STAT    cell IO uncompressed bytes                                             682,672,128
STAT    cell blocks processed by cache layer                                        83,949
STAT    cell commit cache queries                                                   83,949
STAT    cell flash cache read hits                                                   1,269
STAT    cell num smartio automem buffer allocation attempts                              1
STAT    cell physical IO bytes eligible for predicate offload                  682,672,128
STAT    cell physical IO interconnect bytes                                    682,907,280
STAT    cell physical IO interconnect bytes returned by smart scan             682,907,280
STAT    cell scans                                                                       1
STAT    cleanout - number of ktugct calls                                           83,334
STAT    cleanouts and rollbacks - consistent read gets                              83,334
STAT    consistent changes                                                             791
STAT    consistent gets                                                            917,053
STAT    consistent gets direct                                                      83,334
STAT    consistent gets examination                                                833,339
STAT    consistent gets examination (fastpath)                                      83,337
STAT    consistent gets from cache                                                 833,719
STAT    consistent gets pin                                                            380
STAT    consistent gets pin (fastpath)                                                 380
STAT    data blocks consistent reads - undo records applied                        750,002
STAT    immediate (CR) block cleanout applications                                  83,334
STAT    physical read IO requests                                                    1,278
STAT    physical read bytes                                                    682,672,128
STAT    physical read requests optimized                                             1,269
STAT    physical read total IO requests                                              1,278
STAT    physical read total bytes                                              682,672,128
STAT    physical read total bytes optimized                                    678,494,208
STAT    physical read total multi block requests                                       885
STAT    physical reads                                                              83,334
STAT    physical reads direct                                                       83,334
STAT    table fetch by rowid                                                             1
STAT    table scan blocks gotten                                                    83,334
STAT    table scan disk non-IMC rows gotten                                        500,000
STAT    table scan rows gotten                                                     500,000
STAT    table scans (direct read)                                                        1


------------------------------------------------------------------------------------------
3. About
------------------------------------------------------------------------------------------
- MyStats v2.01 by Adrian Billington (http://www.oracle-developer.net)
- Based on the SNAP_MY_STATS utility by Jonathan Lewis

==========================================================================================
End of report
==========================================================================================

In the example table scans (direct read) indicates that the segment was read using a direct path read. Since this example was executed on an Exadata this direct path read was performed as a Smart Scan (cell scans = 1). However, the Smart Scan was not very successful: although all 83,334 table blocks were opened by the cache layer (the first one to touch a block on the cell during the Smart Scan) none of them passed the examination of the transaction layer-they have all been discarded. You can see that there was no saving by using the Smart Scan here: cell physical IO bytes eligible for predicate offload is equal to cell physical IO interconnect bytes returned by smart scan.

83,334 blocks were read during the table scan (table scan blocks gotten), all of which were read directly (physical reads direct). All of the blocks read had an active transaction (active txn count during cleanout) and all of these had to be rolled back (cleanouts and rollbacks – consistent read gets).

The same amount of work has to be done for every read.

Commit in session 1

After committing the transaction in session 1 the statistics for the select statement in session 2 are as follows (also note the execution time)

SQL> select count(*) from t1_100k;

  COUNT(*)
----------
    500000

Elapsed: 00:00:00.16

SQL> @scripts/mystats stop r=physical|cleanout|consistent|cell|table

------------------------------------------------------------------------------------------
2. Statistics Report
------------------------------------------------------------------------------------------

Type    Statistic Name                                                               Value
------  ----------------------------------------------------------------  ----------------
STAT    cell IO uncompressed bytes                                             682,672,128
STAT    cell blocks helped by commit cache                                          83,334
STAT    cell blocks processed by cache layer                                        83,334
STAT    cell blocks processed by data layer                                         83,334
STAT    cell blocks processed by txn layer                                          83,334
STAT    cell commit cache queries                                                   83,334
STAT    cell flash cache read hits                                                     663
STAT    cell num smartio automem buffer allocation attempts                              1
STAT    cell physical IO bytes eligible for predicate offload                  682,672,128
STAT    cell physical IO interconnect bytes                                     13,455,400
STAT    cell physical IO interconnect bytes returned by smart scan              13,455,400
STAT    cell scans                                                                       1
STAT    cell transactions found in commit cache                                     83,334
STAT    consistent changes                                                             791
STAT    consistent gets                                                             83,717
STAT    consistent gets direct                                                      83,334
STAT    consistent gets examination                                                      3
STAT    consistent gets examination (fastpath)                                           3
STAT    consistent gets from cache                                                     383
STAT    consistent gets pin                                                            380
STAT    consistent gets pin (fastpath)                                                 380
STAT    physical read IO requests                                                      663
STAT    physical read bytes                                                    682,672,128
STAT    physical read requests optimized                                               663
STAT    physical read total IO requests                                                663
STAT    physical read total bytes                                              682,672,128
STAT    physical read total bytes optimized                                    682,672,128
STAT    physical read total multi block requests                                       654
STAT    physical reads                                                              83,334
STAT    physical reads direct                                                       83,334
STAT    table fetch by rowid                                                             1
STAT    table scan blocks gotten                                                    83,334
STAT    table scan disk non-IMC rows gotten                                        500,000
STAT    table scan rows gotten                                                     500,000
STAT    table scans (direct read)                                                        1
STAT    table scans (short tables)                                                       1


------------------------------------------------------------------------------------------
3. About
------------------------------------------------------------------------------------------
- MyStats v2.01 by Adrian Billington (http://www.oracle-developer.net)
- Based on the SNAP_MY_STATS utility by Jonathan Lewis

==========================================================================================
End of report
==========================================================================================

There are no more active transactions found during the cleanout. The blocks however still have to be cleaned out every time the direct path read is performed. As a matter of design principle Oracle cannot reuse the blocks it has read with a direct path read (or Smart Scan for that matter) they simply are not cached.

But have a look at the execution time: quite a nice performance improvement that is. In fact there are a few things worth mentioning:

  • The segment is successfully scanned via a Smart Scan. How can you tell?
    • cell scans = 1
    • cell physical IO interconnect bytes returned by smart scan is a lot lower than cell physical IO bytes eligible for predicate offload
    • You see cell blocks processed by (cache|transaction|data) layer matching the number of blocks the table is made up of. Previously you only saw blocks processed by the cache layer
  • The commit cache made this happen
    • cell commit cache queries
    • cell blocks helped by commit cache
    • cell transactions found in commit cache

The commit cache “lives” in the cells and probably caches recently committed transactions. If a block is read by the cell and an active transaction is found the cell has a couple of options to avoid a round-trip of the block to the RDBMS layer for consistent read processing:

  1. If possible it uses the minscn optimisation. Oracle tracks the minimum SCN of all active transactions. The cell software can compare the SCN from the ITL in the block with the lowest SCN of all active transactions. If that number is greater than the SCN found in the block the transaction must have already completed and it is safe to read the block. This optimisation is not shown in the blog post, you’d see  cell blocks helped by minscn optimization increase
  2. If the first optimisation does not help the cells make use of the commit cache-visible as cell commit cache queries and if a cache hit has been scored, cell blocks helped by commit cache. While the transaction in session 1 hasn’t completed yet you could only see the commit cache queries. After the commit the queries were successful

If both of them fail the block must be sent to the RDBMS layer for consistent read processing. In that case there is no difference to the treatment of direct path read blocks outside Exadata.

Summary

The commit cache is a great enhancement Exadata offers for databases using the platform. While non-Exadata deployments will have to clean out blocks that haven’t been subject to cleanouts every time the “dirty” block is read via direct path read, this can be avoided on Exadata if the transaction has committed.

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