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Home -> Community -> Mailing Lists -> Oracle-L -> RE: CONSISTANT GETS
Naveen,
A CU block is fetched "as-is"; whatever's in the block at the time of the read is what comes back from the LIO. CU mode is used when read consistency is not an issue; for example, when reading your own (private) sort blocks. I think if you DELETE without a WHERE clause, you'll see CU blocks (not sure).
Reading in CR mode includes potentially more codepath that can reconstruct the block as of a specified point in history. It's how the Oracle kernel does read consistency. The mechanism includes checking the ITL in the header of the block being read to understand whether the version of the block is appropriate for the SCN of the query requesting the block. It goes kind of like this:
while SCN of block is newer than SCN of query {
clone (i.e., copy) the block in the buffer cache;
LIO the undo block referenced by the most recent ITL entry (i.e., LIO
a rollback segment in CR mode);
apply the undo to the block (resulting in a new, older SCN for the
block);
}
I've never known whether the maximum number of clone operations per CR read is the number of undo applications, or maybe it's just 1. The algorithm might actually be this:
if SCN of block is newer than SCN of query then clone the block;
while SCN of block is newer than SCN of query {
LIO the undo block referenced by the most recent ITL entry;
Apply the undo to the block;
}
Jonathan Lewis was once on the trail of figuring this out, but I'm not sure he ever satisfied himself with an answer. Has anyone on the list constructed a test to reveal the answer?
Cary Millsap
Hotsos Enterprises, Ltd.
http://www.hotsos.com
Upcoming events:
- Hotsos Clinic, Dec 9-11 Honolulu
-----Original Message-----
Nahata
Sent: Friday, November 15, 2002 6:48 AM
To: Multiple recipients of list ORACLE-L
Cary,
I read your wonderful article. What is the exact difference between CR
and
CU, blocks fetched in Consistent and Current mode?
Regards
Naveen
-----Original Message-----
Sent: Friday, November 15, 2002 2:54 PM
To: Multiple recipients of list ORACLE-L
It's the count of a certain type of fetch operations of blocks from the database buffer cache. See "Why you should focus on LIOs instead of PIOs" at www.hotsos.com/catalog for details.
Cary Millsap
Hotsos Enterprises, Ltd.
http://www.hotsos.com
Upcoming events:
- Hotsos Clinic, Dec 9-11 Honolulu
-----Original Message-----
Alavi
Sent: Thursday, November 14, 2002 2:10 PM
To: Multiple recipients of list ORACLE-L
Sorry for asking such a obvious question, but CONSISTANT GETS means
calling
rows from Database????
Thanks,
-----Original Message-----
Sent: Thursday, November 14, 2002 10:35 AM
To: Multiple recipients of list ORACLE-L
Hamid,
I'm sorry: Unless your SQL returns fewer than about 800,000 rows to the calling application (or an aggregation of 800,000 rows), then the statement "we have done all the necessary tuning on all the SQL queries" is not yet true.
If your SQL does actually return about 800,000 rows, then it is time to begin thinking about the mismatch between business processing requirements and the logical structure of your data.
The answer to your problem is not in your instance parameters.
Cary Millsap
Hotsos Enterprises, Ltd.
http://www.hotsos.com
Upcoming events:
- Hotsos Clinic, Dec 9-11 Honolulu
-----Original Message-----
Alavi
Sent: Thursday, November 14, 2002 11:50 AM
To: Multiple recipients of list ORACLE-L
Dear List,
I am monitoring a database, & I findout there is a transaction which
runing
a long time and others are waiting for this transaction, this
transaction
have 8,000,000 consistant gets with only 1 Physical I/O.
My question is, what I have to do except the SQL tuning to make this
transaction faster, we have done all the necessary tuning on all the SQL
query's.
Here is a copy of ora.ini:
Oracle 8.1.7.4 on sun solaris 2.8
background_dump_dest = /oracle/admin/cmstst/bdump compatible = 8.1.7.4
control_files = "/cmsdb/cmstst/control02.ctl" control_files = "/oralogs1/cmstst/control03.ctl" control_files = "/oracle/oradata/cmstst/control01.ctl" core_dump_dest = /oracle/admin/cmstst/cdump db_block_buffers = 10000 ??? this need toincrease?
db_block_lru_latches = 4 db_block_size = 8192 db_file_multiblock_read_count = 16 db_name = cmstst hash_area_size = 2048000 ??? need tuning???
log_archive_dest_1 = "location=/archlogs/cmstst" log_archive_format = "arch%s.arc" log_archive_start = TRUE log_buffer = 262144 ?? this log
log_checkpoint_interval = 10000 ?? log_checkpoint_timeout = 1800 max_enabled_roles = 30
I realy appreciate your help and assistant. I am getting confused, just
want
to know changing any of these parameter help the performance to reduce
the
number of CONSISTANT GETS or NOT???
Thanks in advance.
Hamid Alavi
Office 818 737-0526
Cell 818 416-5095
using the information. Please contact the sender immediately by return e-mail and delete the original message from your system.
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-- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: Hamid Alavi INET: hamid.alavi_at_quovadx.com Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing). -- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: Cary Millsap INET: cary.millsap_at_hotsos.com Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing). ======================= Confidentiality Statement ======================= The information contained in this message and any attachments is intended only for the use of the individual or entity to which it is addressed, and may contain information that is PRIVILEGED, CONFIDENTIAL and exempt from disclosure under applicable law. If you have received this message in error, you are prohibited from copying, distributing, or using the information. Please contact the sender immediately by return e-mail and delete the original message from your system. ===================== End Confidentiality Statement ===================== -- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: Hamid Alavi INET: hamid.alavi_at_quovadx.com Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing). -- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: Cary Millsap INET: cary.millsap_at_hotsos.com Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing). -- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: Naveen Nahata INET: naveen_nahata_at_mindtree.com Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing). -- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: Cary Millsap INET: cary.millsap_at_hotsos.com Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).Received on Fri Nov 15 2002 - 13:08:44 CST
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