Re: block change tracking buffer space !

  • From: Jonathan Lewis <jonathan@xxxxxxxxxxxxxxxxxx>
  • To: ORACLE-L <Oracle-L@xxxxxxxxxxxxx>
  • Date: Tue, 13 Mar 2018 16:43:29 +0000


Nitin,

I think I'd look at a couple of other figures before trying to fiddle with 
undocumented parameters.

What does the wait event histogram say about change tracking writes and reads ? 
Any clues that there's an I/O problem there ?

How about checking the size and free space in the large pool - changing the 
allocation size may make no difference if the large pool can't supply it. Maybe 
setting a larger large_pool_size would be sufficient.

It's a long time since I looked at BCT, but maybe your data files are growing 
and this is putting some odd pressure on your change tracking file that's 
causing delays with writes to the file.


Regards
Jonathan Lewis



________________________________________
From: oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> on behalf 
of Nitin Saxena <ntnsxn7@xxxxxxxxx>
Sent: 13 March 2018 00:48
To: ORACLE-L
Subject: block change tracking buffer space !

Hello Everyone,

I am facing lot of concurrency wait events due to "block change tracking buffer 
space"  when rman L0 backup is run. I see that database gets overwhelmed with 
concurrency due to  less space for CTWR buffer as shown in event. I see 
following problem area . Any recommendations if we should bump 
"_bct_public_dba_buffer_size" to higher value ?





NAME                                          VALUE                          
DESCRIPTION
--------------------------------------------- ------------------------------ 
-------------------------------------------------------
_bct_public_dba_buffer_size                   0                              
total size of all public change tracking dba buffers,
                                                                             in 
bytes


NAME                                          VALUE                          
DESCRIPTION
--------------------------------------------- ------------------------------ 
-------------------------------------------------------
_bct_buffer_allocation_max                    104857600                      
maximum size of all change tracking buffer allocations,
                                                                             in 
bytes


SQL> select dba_buffer_count_public*dba_entry_count_public*dba_entry_size from 
X$KRCSTAT;

DBA_BUFFER_COUNT_PUBLIC*DBA_ENTRY_COUNT_PUBLIC*DBA_ENTRY_SIZE
-------------------------------------------------------------
                                                     25276400




SQL> SELECT * FROM v$sgastat WHERE name like 'CTWR%';

POOL         NAME                            BYTES
------------ -------------------------- ----------
large pool   CTWR dba buffer              11386880


snippet from ash and dash is as follows:

%This     SECONDS WAIT_CHAIN
------ ---------- 
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  12%       14650 -> 1vyayutd1117p:buffer busy waits -> 1vyayutd1117p:block 
change tracking buffer space


--
Cheers
Nitin
--
http://www.freelists.org/webpage/oracle-l


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