Introduction
When using direct path read, oracle reads the blocks from the disk. The buffer cache is not being used.
Then, when oracle begins to read a segment using direct path read, it flush its dirty blocks to disk (A dirty block is a block that does not have the same image in memory and on disk). During that time, the session that triggered the direct path read is waiting for the “enq: KO – fast object checkpoint” wait event.
In this post, I just want to see with my own eyes that the dirty blocks being flushed to disk belong only to the segment being read.
Tests
For the test, I used two tables on a 11.2.0.4 database. One table is made of 2 partitions and one is not partitioned (then 3 segments):
BDT:BDT1> select SEGMENT_NAME,PARTITION_NAME from dba_segments where segment_name in ('BDT1','BDT2');
SEGMENT_NAME PARTITION_NAME
------------ --------------
BDT2
BDT1 P2
BDT1 P1
The rows distribution across segments, blocks and file relative numbers is the following:
BDT:BDT1> l 1 select u.object_name,u.subobject_name,dbms_rowid.rowid_block_number(BDT1.rowid) BLOCK_NUMBER, dbms_rowid.rowid_relative_fno(BDT1.rowid) FNO 2 from BDT1, user_objects u 3 where dbms_rowid.rowid_object(BDT1.rowid) = u.object_id 4 union all 5 select u.object_name,u.subobject_name,dbms_rowid.rowid_block_number(BDT2.rowid) BLOCK_NUMBER, dbms_rowid.rowid_relative_fno(BDT2.rowid) FNO 6 from BDT2, user_objects u 7 where dbms_rowid.rowid_object(BDT2.rowid) = u.object_id 8 order by 1,2 9* BDT:BDT1> / OBJECT_NAME SUBOBJECT_NAME BLOCK_NUMBER FNO ------------ --------------- ------------ ----------- BDT1 P1 2509842 5 BDT1 P1 2509842 5 BDT1 P1 2509842 5 BDT1 P1 2509842 5 BDT1 P2 2510866 5 BDT1 P2 2510866 5 BDT1 P2 2510866 5 BDT1 P2 2510866 5 BDT1 P2 2510866 5 BDT2 1359091 5 BDT2 1359091 5 BDT2 1359091 5 BDT2 1359091 5 BDT2 1359091 5 BDT2 1359091 5 BDT2 1359091 5 BDT2 1359091 5 BDT2 1359091 5 18 rows selected.
so that all the rows of the segment:
- BDT1:P1 are in datafile 5, block 2509842
- BDT1:P2 are in datafile 5, block 2510866
- BDT2 are in datafile 5, block 1359091
Remark:
BDT1:P<N> stands for table BDT1 partition <N>
Now, let’s update both tables (without committing):
BDT:BDT1> update BDT1 set ID2=100; 9 rows updated. BDT:BDT1> update BDT2 set ID2=100; 9 rows updated.
As I do not commit (nor rollback), then those 3 blocks in memory contain an active transaction.
In another session, let’s force serial direct path read and read partition P1 from BDT1 only:
BDT:BDT1> alter session set "_serial_direct_read"=always; Session altered. BDT:BDT1> select count(*) from BDT1 partition (P1); COUNT(*) ----------- 4
Now, check its wait events and related statistics with snapper. A cool thing with snapper is that I can get wait events and statistics from one tool.
SYS:BDT1> @snapper all,end 5 1 20
Sampling SID 20 with interval 5 seconds, taking 1 snapshots...
-- Session Snapper v4.22 - by Tanel Poder ( http://blog.tanelpoder.com/snapper ) - Enjoy the Most Advanced Oracle Troubleshooting Script on the Planet! 🙂
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SID @INST, USERNAME , TYPE, STATISTIC , DELTA, HDELTA/SEC, %TIME, GRAPH , NUM_WAITS, WAITS/SEC, AVERAGES
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
20 @1, BDT , STAT, physical reads direct , 1, .02, , , , , .5 per execution
20 @1, BDT , WAIT, enq: KO - fast object checkpoint , 1117, 18.12us, .0%, [ ], 1, .02, 1.12ms average wait
-- End of Stats snap 1, end=2015-04-29 08:25:20, seconds=61.7
-- End of ASH snap 1, end=, seconds=, samples_taken=0, AAS=(No ASH sampling in begin/end snapshot mode)
PL/SQL procedure successfully completed.
For brevity and clarity, I keep only the wait events and statistics of interest.
As we can see, the direct path read has been used and the “fast object checkpoint” occurred (as the session waited for it).
Now let’s check which block(s) has/have been flushed to disk: to do so, I’ll dump those 3 blocks from disk and check if they contain an active transaction (as the ones in memory).
BDT:BDT1> !cat dump_blocks.sql -- BDT2 alter system dump datafile 5 block 1359091; -- BDT1:P2 alter system dump datafile 5 block 2510866; -- BDT1:P1 alter system dump datafile 5 block 2509842; select value from v$diag_info where name like 'Default%'; BDT:BDT1> @dump_blocks.sql System altered. System altered. System altered. VALUE ------------------------------ /u01/app/oracle/diag/rdbms/bdt/BDT1/trace/BDT1_ora_91815.trc BDT:BDT1> !view /u01/app/oracle/diag/rdbms/bdt/BDT1/trace/BDT1_ora_91815.trc
The Interested Transaction List (ITL) for the first block that has been dumped from disk (BDT2 Table) looks like:
Itl Xid Uba Flag Lck Scn/Fsc 0x01 0xffff.000.00000000 0x00000000.0000.00 C--- 0 scn 0x0000.0016f9c7 0x02 0x0000.000.00000000 0x00000000.0000.00 ---- 0 fsc 0x0000.00000000 0x03 0x0000.000.00000000 0x00000000.0000.00 ---- 0 fsc 0x0000.00000000
We can see that the block on disk does not contain an active transaction in it (There is no rows locked as Lck=0). We can conclude that the block that belongs to the BDT2 table has not been flushed to disk (The block is still dirty).
The Interested Transaction List (ITL) for the second block that has been dumped from disk (BDT1:P2) looks like:
Itl Xid Uba Flag Lck Scn/Fsc 0x01 0xffff.000.00000000 0x00000000.0000.00 C--- 0 scn 0x0000.0016f99f 0x02 0x0000.000.00000000 0x00000000.0000.00 ---- 0 fsc 0x0000.00000000 0x03 0x0000.000.00000000 0x00000000.0000.00 ---- 0 fsc 0x0000.00000000
We can see that the block on disk does not contain an active transaction in it (There is no rows locked as Lck=0). We can conclude that the block that belongs to the partition P2 of the BDT1 table has not been flushed to disk (The block is still dirty).
The Interested Transaction List (ITL) for the third block that has been dumped from disk (BDT1:P1) looks like:
Itl Xid Uba Flag Lck Scn/Fsc 0x01 0xffff.000.00000000 0x00000000.0000.00 C--- 0 scn 0x0000.0016f998 0x02 0x000a.005.00001025 0x00c00211.038e.23 ---- 4 fsc 0x0000.00000000 0x03 0x0000.000.00000000 0x00000000.0000.00 ---- 0 fsc 0x0000.00000000
We can see that the block on disk does contain an active transaction in it (4 rows are locked as Lck=4). This is the block that has been read in direct path read during the select on BDT1:P1.
We can conclude that the block that belongs to BDT1:P1 has been flushed to disk (The block is not dirty anymore).
Conclusion
When oracle begins to read a segment using direct path read:
- It does not flush to disk the dirty blocks from other segments.
- Only the dirty blocks of the segment being read are flushed to disk.
bdt's blog 
Very cool and clear demonstration Bertrand ! Have an additional question : Since the transaction is not commited i guess that when the direct path read is done oracle detects it has to read the previous version from IMU. Do you have any idea if this fast checkpoint mechanism also applies to IMU (IMU flushed to undo segment ?). Thanks !
Thanks!
The undo blocks have not been flushed to disk during my test. You can check this by counting the number of dirty blocks into the UNDO tablespace that way:
SYS:BDT1> l
1 select
2 tablespace_name, count(*)
3 from
4 v$bh, dba_data_files
5 where
6 dirty = ‘Y’
7 and file#=file_id
8 group by tablespace_name
9 order by
10 count(*)
I can see the same number of dirty blocks into the UNDO tablespace before and after the direct path read.
Bertrand
Thanks a lot for the follow up and the tip Bertrand !
Hi Bertrand
How does Oracle interpret reads from smart cache. What wait event is reported when reading dirty buffers that have been flushed to smart cache
Hello,
What smart cache? database smart cache or exadata smart flash cache?
Bertrand
Very nice article !! Thanks for the demo.