Dynamic tracing of Oracle logical I/O

I would like to provide one more option to investigate Oracle logical I/O with DTrace.
This method allows to see each consistent/current gets with details about block, object and location from which function is called.

This is the DTrace script: dtracelio.d
Note: Dtrace LIO with new features is released

Short description:
This tool allows to see:
– details of each call (consistent/current get)
– details of blocks been read
– function from which call been performed without investigating of call stack (“where”)
– aggregation of performed calls

The tool can be especially helpful in 11g because x$kcbuwhy (x$kcbsw in 10g) is not populated properly there.
Note: Examples of using views x$kcbsw/x$kcbuwhy:
– Investigating Logical I/O by Jonathan Lewis
– Finding the reasons for excessive logical IOs by Tanel Poder

Current version of the script monitors execution of two Kernel Cache Layer functions:
kcbgtcr – Kernel Cache Buffer Get Consistent Read. Number of calls of this function we can see as statistic “consistent gets” and in 10046 trace as “cr=”.
kcbgcur – Kernel Cache Buffer Get Current Read. Number of calls of this function we can see as statistic “db block gets” in 10046 trace as “cu=”.
upd:
statistic “db block gets” is incremented not only by kcbgcur, but also by kcbget (included in current version of dtracelio), kcbnew (coming soon), kcblnb* (direct-path load) and some others. But kcbgcur is most often used.

In the both functions:
– the first argument is a pointer on the structure which describes a block;
– the second argument is unknown, but I could suggest that this is lock_mode (from MOS bug 7109078);
– the third argument (the least significant bits) is “where” (x$kcbwh), that is a module where the function is called.

Usage:
dtracelio.d PID [show_each_call]

PID – unix process ID
show_each_call – if 0 then output of each call will be disabled, else details of each call will be shown

A little example of usage. Let’s try to see what is going on during select on very small table:

SQL> @spid
 
       SID ORACLE_DEDICATED_PROCESS CLIENTPID
---------- ------------------------ ------------------------
        41 18949                    900:408
 
SQL> create table dualcopy as select * from dual;
 
Table created
 
SQL> select * from dualcopy;
 
DUMMY
-----
X

Notice, that I have executed “select * from dualcopy” to avoid hard parsing in consequences executions.

Let’s execute dtracelio

dtracelio.d 18949

I have executed the script with default parameter show_each_call, it means that details of each calls will be shown.

Now I am executing our query:

SQL> select * from dualcopy;
 
DUMMY
-----
X

And this is an output of dtracelio

kcbgtcr(0xFFFFFD7FFFDFB0F0,0,742,0) [tsn: 4 rdba: 0x100060a (4/1546) obj: 79218 dobj: 79218] where: 742
kcbgtcr(0xFFFFFD7FFFDFAED0,0,743,0) [tsn: 4 rdba: 0x100060a (4/1546) obj: 79218 dobj: 79218] where: 743
kcbgtcr(0xFFFFFD7FFDC3B2E0,0,860,0) [tsn: 4 rdba: 0x100060b (4/1547) obj: 79218 dobj: 79218] where: 860

You can see here:
kcbgtcr(0xFFFFFD7FFFDFB0F0,0,742,0) – it is function call with 3 arguments,
tsn: 4 – a tablespace number, ts# from v$tablespace
rdba: 0x100060a – a relative dba (data block address)
(4/1546) – file 4 block 1546
obj: 79218 – dictionary object number, object_id from dba_objects
dobj: 79218 – data object number, data_object_id from dba_objects
where: 742 – location from function (kcbgtcr in this case) was executed. This is INDX from x$kcbwf.

SQL> select indx, kcbwhdes from x$kcbwh where indx in (742, 743, 860);
 
      INDX KCBWHDES
---------- ----------------------------------------------------------------
       742 ktewh25: kteinicnt
       743 ktewh26: kteinpscan
       860 kdswh01: kdstgr

It is functions from which our function kcbgtcr was executed. It is parent function which you will see if will print call stack during the call of kcbgtcr.
If you would print call stack you will see (I’ve just added ustack() in the script):

kcbgtcr(0xFFFFFD7FFFDFB0F0,0,742,0) [tsn: 4 rdba: 0x100060a (4/1546) obj: 79218 dobj: 79218] where: 742

              oracle`kcbgtcr
              oracle`ktecgshx+0x3f6
              oracle`kteinicnt1+0x1cf
              oracle`qertbFetch+0xc2a
              oracle`opifch2+0xaa2
              oracle`opifch+0x3a
              oracle`opiodr+0x433
              oracle`ttcpip+0x599
              oracle`opitsk+0x600
              oracle`opiino+0x675
              oracle`opiodr+0x433
              oracle`opidrv+0x32e
              oracle`sou2o+0x57
              oracle`opimai_real+0x219
              oracle`ssthrdmain+0x14e
              oracle`main+0xcb
              oracle`0x159e67c

kcbgtcr(0xFFFFFD7FFFDFAED0,0,743,0) [tsn: 4 rdba: 0x100060a (4/1546) obj: 79218 dobj: 79218] where: 743

              oracle`kcbgtcr
              oracle`ktecgshx+0x3f6
              oracle`kteinpscan+0x1b6
              oracle`kteiniscan+0x2c
              oracle`kdselini+0x2f
              oracle`kdsirs1+0x792
              oracle`kdsirs+0x2b
              oracle`qertbFetch+0xbb0
              oracle`opifch2+0xaa2
              oracle`opifch+0x3a
              oracle`opiodr+0x433
              oracle`ttcpip+0x599
              oracle`opitsk+0x600
              oracle`opiino+0x675
              oracle`opiodr+0x433
              oracle`opidrv+0x32e
              oracle`sou2o+0x57
              oracle`opimai_real+0x219
              oracle`ssthrdmain+0x14e
              oracle`main+0xcb

kcbgtcr(0xFFFFFD7FFDC3B2E0,0,860,0) [tsn: 4 rdba: 0x100060b (4/1547) obj: 79218 dobj: 79218] where: 860

              oracle`kcbgtcr
              oracle`ktrget2+0x27d
              oracle`kdstgr+0x46b
              oracle`qertbFetch+0x466
              oracle`opifch2+0xaa2
              oracle`opifch+0x3a
              oracle`opiodr+0x433
              oracle`ttcpip+0x599
              oracle`opitsk+0x600
              oracle`opiino+0x675
              oracle`opiodr+0x433
              oracle`opidrv+0x32e
              oracle`sou2o+0x57
              oracle`opimai_real+0x219
              oracle`ssthrdmain+0x14e
              oracle`main+0xcb
              oracle`0x159e67c

So, 3 consistent gets, 3 logical reads were performed.
Two times block (4/1546) – this is segment header – has been read. The first time from function kteinicnt and the second time from function kteinpscan.
And the third get is block (4/1547) – this is data block – was read from function kdstgr.

After that, if you push Ctrl+C, dtracelio script will be finished, and you will see Summary section:

^C

========================= Summary ==========================
object_id    data_object_id  function     where        count
79218        79218           kcbgtcr      742          1
79218        79218           kcbgtcr      743          1
79218        79218           kcbgtcr      860          1

Summary section is aggregation of all calls grouped by object_id, data_object_id, function, where.

example 2
There was question on sql.ru. The author asked “why number of current gets (db block gets) during update in distributed transaction is more then in usual, non-distributed transaction?”.
In his case it were:
3070178 current gets in non-distributed transaction
6070211 current gets in distributed transaction

So, let’s find out what exactly is read during distributed transaction and is not read in usual, non-distributed transaction.

SQL> select * from v$version;
 
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
PL/SQL Release 11.2.0.2.0 - Production
CORE	11.2.0.2.0	Production
TNS for Solaris: Version 11.2.0.2.0 - Production
NLSRTL Version 11.2.0.2.0 - Production

SQL> @spid
 
       SID ORACLE_DEDICATED_PROCESS CLIENTPID
---------- ------------------------ ------------------------
         1 16727                    900:408

SQL> create table test as select * from all_objects;
 
Table created

SQL> alter session set optimizer_dynamic_sampling=0;
 
Session altered

SQL> @10046
 
Session altered

Here we are executing dtracelio.d:

dtracelio.d 16727 0

Here I have asked to show me only summary section of executions of functions performing logical I/O by process 16727.

1st update, without distributed transaction

SQL> begin
  2      update test set owner = owner;
  3      commit;
  4  end;
  5  /
 
PL/SQL procedure successfully completed

After the update has been completed we can push Ctrl+C to finish the DTrace and see the Summary section:

^C
========================= Summary ==========================
object_id    data_object_id  function     where        count
...
0            -1              kcbgcur      48           16
0            -1              kcbgcur      156          16
17           17              kcbgtcr      869          18
0            -1              kcbgcur      51           253
0            -1              kcbgcur      79           1365
79204        79204           kcbgcur      878          1656
0            -1              kcbgcur      50           2357
-1           79204           kcbgcur      859          2357
79204        79204           kcbgtcr      869          2726
79204        79204           kcbgcur      1053         75105

An excerpt from raw 10046 trace:

=====================
PARSING IN CURSOR #18446741324891442008 len=60 dep=0 uid=0 oct=47 lid=0 tim=229288260276 hv=2516922591 ad='88f557c0' sqlid='6sj578yb0ac6z'
begin
    update test set owner = owner;
    commit;
end;
END OF STMT
....
EXEC #18446741324891442008:c=2780000,e=4884618,p=0,cr=2776,cu=84545,mis=0,r=1,dep=0,og=1,plh=0,tim=229293144944

Pay attention, cu=84545.

Now let’s execute the same update within distributed transaction

SQL> declare
  2    i integer;
  3  begin
  4  
  5    select count(*) into i from dual@dblink;
  6    update test set owner = owner;
  7    commit;
  8  
  9  end;
 10  /

PARSING IN CURSOR #18446741324891442008 len=122 dep=0 uid=0 oct=47 lid=0 tim=229346672450 hv=1954107067 ad='88f04bc0' sqlid='arss8t9u7kmpv'
declare
  i integer;
begin
  select count(*) into i from dual@dblink;
  update test set owner = owner;
  commit;
end;
END OF STMT
...
EXEC #18446741324891442008:c=2950000,e=3432722,p=0,cr=2298,cu=159479,mis=0,r=1,dep=0,og=1,plh=0,tim=229350105247

As we can see cu=159479, about in two times more then in previous execution.

dtracelio.d output:

========================= Summary ==========================
object_id    data_object_id  function     where        count
...
0            -1              kcbgcur      48           16
0            -1              kcbgcur      156          16
17           17              kcbgtcr      869          18
0            -1              kcbgcur      51           49
0            -1              kcbgcur      50           442
-1           79204           kcbgcur      859          442
0            -1              kcbgcur      79           1161
79204        79204           kcbgcur      878          1177
79204        79204           kcbgtcr      869          2248
79204        79204           kcbgcur      1053         75105
0            -1              kcbgcur      109          79853

Pay attention on the bottom row

0            -1              kcbgcur      109          79853

79853 additional db block gets of some undo block which does not exist in previous output.

If we take a look at the output of dtracelio.d with enabled output of each call we will see something like this:

...
kcbgcur(0xFFFFFD7FFDC42D68,2,1053,0) [tsn: 0 rdba: 0x4186e6 (1/100070) obj: 79204 dobj: 79204] where: 1053
kcbgcur(0xFFFFFD7FFFDF46A0,1,109,0) [tsn: 2 rdba: 0xc000f0 (3/240) obj: 0 dobj: -1] where: 109
kcbgcur(0xFFFFFD7FFDC42D68,2,1053,0) [tsn: 0 rdba: 0x4186e6 (1/100070) obj: 79204 dobj: 79204] where: 1053
kcbgcur(0xFFFFFD7FFFDF46A0,1,109,0) [tsn: 2 rdba: 0xc000f0 (3/240) obj: 0 dobj: -1] where: 109
kcbgcur(0xFFFFFD7FFDC42D68,2,1053,0) [tsn: 0 rdba: 0x4186e6 (1/100070) obj: 79204 dobj: 79204] where: 1053
kcbgcur(0xFFFFFD7FFFDF46A0,1,109,0) [tsn: 2 rdba: 0xc000f0 (3/240) obj: 0 dobj: -1] where: 109
kcbgcur(0xFFFFFD7FFDC42D68,2,1053,0) [tsn: 0 rdba: 0x4186e6 (1/100070) obj: 79204 dobj: 79204] where: 1053
kcbgcur(0xFFFFFD7FFFDF46A0,1,109,0) [tsn: 2 rdba: 0xc000f0 (3/240) obj: 0 dobj: -1] where: 109
kcbgcur(0xFFFFFD7FFDC42D68,2,1053,0) [tsn: 0 rdba: 0x4186e6 (1/100070) obj: 79204 dobj: 79204] where: 1053
kcbgcur(0xFFFFFD7FFFDF46A0,1,109,0) [tsn: 2 rdba: 0xc000f0 (3/240) obj: 0 dobj: -1] where: 109
kcbgcur(0xFFFFFD7FFDC42D68,2,1053,0) [tsn: 0 rdba: 0x4186e6 (1/100070) obj: 79204 dobj: 79204] where: 1053
...

Here we can see these additional calls “where: 109” and that the same block (3/240) is read during these calls.
Here is only excerpt, but I checked, all 79853 calls for the same block from the same “where” (109 in this case).
Update: updated version of DTraceLIO allows to see that these current gets were performed in shared mode, so content of the block is not changed.

Let’s try to look at inside the block.

alter system dump datafile 3 block 240;

An excerpt from block dump:

...
Block dump from disk:
buffer tsn: 2 rdba: 0x00c000f0 (3/240)
scn: 0x0000.0015240d seq: 0x02 flg: 0x04 tail: 0x240d2602
frmt: 0x02 chkval: 0x54dc type: 0x26=KTU SMU HEADER BLOCK
...

So, this is the undo header.

What is “where: 109”? “Where” means location in code, let’s try to find function name with indx 109

SQL> select indx, kcbwhdes from x$kcbwh where indx = 109;
 
      INDX KCBWHDES
---------- ----------------------------------------------------------------
       109 ktuwh87: ktugus:ktuGetExtTxnInfo

It is exactly what you will see if print call stack during call with “where: 109” (the second row, function from kcbgcur was called is ktuGetExtTxnInfo):

 
 0000000002c63d01 kcbgcur () + 1
 00000000029272d4 ktuGetExtTxnInfo () + 174
 00000000028f5193 ktugti () + 13
 00000000028e2561 ktuchg2 () + 1971
 0000000002944d2f ktbchg2 () + 12f
 0000000001b04809 kdu_array_flush_retry () + c69
 0000000001b01f42 kdu_array_buf () + 522
 0000000001af5390 kduurp () + 460
 0000000001adea00 kdusru () + 13e0
 0000000001ac6c12 kauupd () + 1b2
 0000000005a3370c updrow () + a5c
 0000000007b8bd27 qerupFetch () + 397
 000000000367cfdc qerstFetch () + 58c
 0000000005a3f2fe updaul () + 4ce
 0000000005a4469d updThreePhaseExe () + 1cd
 0000000005a43abd updexe () + 1ed
 000000000447b2c5 opiexe () + 2455
...

It were for 11.2.0.2. In my 10.2.0.5 all these additional calls are with “where: 383”, “ktuwh02: ktugus” which Jonathan Lewis called “Get Undo Segment header for commit” (Seems it is not correct descriptoin).
And following call stack is during these calls

              oracle`kcbgcur
              oracle`ktugusc+0x321
              oracle`ktugti+0xf2
              oracle`ktuchg+0x139a
              oracle`ktbchg2+0x115
              oracle`kddchg+0x2c2
              oracle`kddlok+0x7da
              oracle`kddlkr+0x16a
              oracle`updrow+0x2417
              oracle`qerupRowProcedure+0x4f
              oracle`qerupFetch+0x339
              oracle`updaul+0x481
              oracle`updThreePhaseExe+0xc72
              oracle`updexe+0x171
              oracle`opiexe+0xf1b
              oracle`opipls+0x98e
              oracle`opiodr+0x433
              oracle`rpidrus+0xde
              oracle`skgmstack+0x80
              oracle`rpidru+0x86

Conclusion
We have determined what exactly are extra current gets within distributed transaction.
Why Oracle 79853 times reads undo header is a topic for further research. I would prefer to write another post about it.

Little oftopic, but interesting point is that Oracle opens distributed transaction (binds undo header, transaction appears in v$transaction, v$global_transaction) for PLSQL block with dblinks during hard parsing. If hard parsing is not performed then during parsing/execution of statement with dblink.
It follows that if you will execute a statement like this

declare
    i integer;
begin
    DML;
    DML;
    DML;
    if 1=0 then
        select count(*) into i from dual@dblink;
    end if;
end;

then either it will be executed within dustributed transaction (with extra current gets) or not depends on will hard parsing be performed or not.

p.s.
Attention!
I am still looking for additional information about
* structure kcbds,
* argments of functions kcbgtcr
* argments of functions kcbgcur
If you have this information please let me know.