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Oracle v$log_history scripts
Oracle Tips by Burleson Consulting
February 26, 2016
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In a busy production environment, it is important to ensure that the
frequency of redo log switches is not more than 5 per hour and the
v$log_history view can help.
As Oracle offloads redo log images
from the redo log filesystem onto the archived redo log filesystem,
excessive I/O can occur and the archived redo log must be promptly
written to tape to keep the free space in the filesystem. If the
archived redo log filesystem becomes full, the database grinds to a
halt.
Other scripts for measuring v$log_history can be found in our
Oracle script collection of over
600 Oracle scripts. I also have details and diagnostic scripts
for Oracle log optimization in
my book "Oracle
Tuning: The Definitive Reference".
Inside log buffer switches
Not many DBA's are aware of the v$log_history view, and how it
can be used to plot the frequency of online redo log switches.
The
Oracle web site offers this query to show log switches by date.
col c1 format a10 heading
"Month"
col c2 format a25 heading "Archive Date"
col c3 format 999 heading "Switches"
compute AVG of C on A
compute AVG of C on REPORT
break on A skip 1 on REPORT skip 1
select
to_char(trunc(first_time), 'Month') c1,
to_char(trunc(first_time), 'Day : DD-Mon-YYYY') c2,
count(*) c3
from
v$log_history
See code depot for full script
where
trunc(first_time) > last_day(sysdate-100) +1
group by
trunc(first_time);
Switching too frequently may sometimes cause performance problems.
The same is true for Oracle RAC databases except that each instance should
have three to four redo log switches per hour. The following script can be
used in Oracle RAC databases to determine the max number of log switches in
any one hour.
-- max_hourly_log_switches.sql
declare
cursor c1 is
select
inst_id,
count(*)
from
gv$log
group by
inst_id,
to_char(first_time,'YYYY-MM-DD HH24')
order by
inst_id;
v_inst number;
v_curr number;
v_cnt number;
v_max number;
v_name varchar2(16);
begin
open c1;
v_curr:=0;
loop
fetch c1 into
v_inst,v_cnt;
exit when
c1%notfound;
if
(v_curr<>v_inst) then
if (v_curr<>0) then
select
instance_name into v_name
from
gv$instance
where
inst_id=v_curr;
See code depot for full script
dbms_output.put_line('Instance: '||v_name||
' Max Hourly Switches: '||v_max);
end if;
v_max := -1;
v_curr := v_inst;
end if;
if (v_max < v_cnt) then
v_max := v_cnt;
end if;
end loop;
select
instance_name into v_name
from
gv$instance
where
inst_id=v_curr;
dbms_output.put_line('Instance:
'||v_name||
' Max Hourly Switches: '||v_max);
end;
/
Instance: orcl1 Max Hourly Switches: 5
Instance: orcl2
Max Hourly Switches: 8
Instance: orcl3 Max Hourly Switches:
3
PL/SQL procedure successfully completed.
The script above will work for any number of instances supporting
the Oracle RAC database. Since each instance has its own thread of redo, we
need to examine the maximum number of log switches per hour in each
instance.
This v$log_history script shows a count and size of the
redo log files by day:
-- Daily Count and Size of Redo
Log Space (Single Instance)
--
SELECT A.*,
Round(A.Count#*B.AVG#/1024/1024) Daily_Avg_Mb
FROM
(
SELECT
To_Char(First_Time,'YYYY-MM-DD') DAY,
Count(1) Count#,
Min(RECID) Min#,
Max(RECID) Max#
FROM
v$log_history
GROUP BY
To_Char(First_Time,'YYYY-MM-DD')
ORDER
BY 1 DESC
) A,
(
SELECT
Avg(BYTES) AVG#,
Count(1) Count#,
Max(BYTES) Max_Bytes,
Min(BYTES) Min_Bytes
FROM
v$log
) B
;
During the RESETLOGS operation, the information in v$log_history and
v$offline_range records are no longer cleared. In addition, two new columns have
been added to indicate the incarnation the records belong to: resetlogs_change#
and resetlogs_time.
select
recid,
thread#,
sequence#,
resetlogs_change#,
resetlogs_time
from
v$log_history
where
rownum < 20;
This script will show the exact times when you get a log switch. You enter the start date and end date
in th script:
break on day skip 1
select
trunc(first_time) day,
substr(to_char(first_time, 'hh24:mi'), 1, 4) || '0' as time,
count(*) switches
from
v$log_history
where
trunc(first_time) between
trunc(to_date('01.02.2016', 'dd.mm.yyyy'))
and
trunc(to_date('29.02.2016', 'dd.mm.yyyy'))
group by
trunc(first_time),
substr(to_char(first_time, 'hh24:mi'), 1, 4)
having
count(*) > 1
order by 1, 2;
The following script can be run to provide a complete log history with the
v$log_history view:
set lines 120;
set pages 999;
SELECT
to_char(first_time,'YYYY-MON-DD') day,
to_char(sum(decode(to_char(first_time,'HH24'),'00',1,0)),'99') "00",
to_char(sum(decode(to_char(first_time,'HH24'),'01',1,0)),'99') "01",
to_char(sum(decode(to_char(first_time,'HH24'),'02',1,0)),'99') "02",
to_char(sum(decode(to_char(first_time,'HH24'),'03',1,0)),'99') "03",
to_char(sum(decode(to_char(first_time,'HH24'),'04',1,0)),'99') "04",
to_char(sum(decode(to_char(first_time,'HH24'),'05',1,0)),'99') "05",
to_char(sum(decode(to_char(first_time,'HH24'),'06',1,0)),'99') "06",
to_char(sum(decode(to_char(first_time,'HH24'),'07',1,0)),'99') "07",
to_char(sum(decode(to_char(first_time,'HH24'),'08',1,0)),'99') "0",
to_char(sum(decode(to_char(first_time,'HH24'),'09',1,0)),'99') "09",
to_char(sum(decode(to_char(first_time,'HH24'),'10',1,0)),'99') "10",
to_char(sum(decode(to_char(first_time,'HH24'),'11',1,0)),'99') "11",
to_char(sum(decode(to_char(first_time,'HH24'),'12',1,0)),'99') "12",
to_char(sum(decode(to_char(first_time,'HH24'),'13',1,0)),'99') "13",
to_char(sum(decode(to_char(first_time,'HH24'),'14',1,0)),'99') "14",
to_char(sum(decode(to_char(first_time,'HH24'),'15',1,0)),'99') "15",
to_char(sum(decode(to_char(first_time,'HH24'),'16',1,0)),'99') "16",
to_char(sum(decode(to_char(first_time,'HH24'),'17',1,0)),'99') "17",
to_char(sum(decode(to_char(first_time,'HH24'),'18',1,0)),'99') "18",
to_char(sum(decode(to_char(first_time,'HH24'),'19',1,0)),'99') "19",
to_char(sum(decode(to_char(first_time,'HH24'),'20',1,0)),'99') "20",
to_char(sum(decode(to_char(first_time,'HH24'),'21',1,0)),'99') "21",
to_char(sum(decode(to_char(first_time,'HH24'),'22',1,0)),'99') "22",
to_char(sum(decode(to_char(first_time,'HH24'),'23',1,0)),'99') "23"
from
v$log_history
GROUP by
to_char(first_time,'YYYY-MON-DD');
Using v$log_history prior to Oracle8i
Prior to Oracle8i the FIRST_TIME column was
named TIME, and this query applies:
select substr(time,1,5) day,
to_char(sum(decode(substr(time,10,2),'00',1,0)),'99') "00",
to_char(sum(decode(substr(time,10,2),'01',1,0)),'99') "01",
to_char(sum(decode(substr(time,10,2),'02',1,0)),'99') "02",
to_char(sum(decode(substr(time,10,2),'03',1,0)),'99') "03",
to_char(sum(decode(substr(time,10,2),'04',1,0)),'99') "04",
to_char(sum(decode(substr(time,10,2),'05',1,0)),'99') "05",
to_char(sum(decode(substr(time,10,2),'06',1,0)),'99') "06",
to_char(sum(decode(substr(time,10,2),'07',1,0)),'99') "07",
to_char(sum(decode(substr(time,10,2),'08',1,0)),'99') "08",
to_char(sum(decode(substr(time,10,2),'09',1,0)),'99') "09",
to_char(sum(decode(substr(time,10,2),'10',1,0)),'99') "10",
to_char(sum(decode(substr(time,10,2),'11',1,0)),'99') "11",
to_char(sum(decode(substr(time,10,2),'12',1,0)),'99') "12",
to_char(sum(decode(substr(time,10,2),'13',1,0)),'99') "13",
to_char(sum(decode(substr(time,10,2),'14',1,0)),'99') "14",
to_char(sum(decode(substr(time,10,2),'15',1,0)),'99') "15",
to_char(sum(decode(substr(time,10,2),'16',1,0)),'99') "16",
to_char(sum(decode(substr(time,10,2),'17',1,0)),'99') "17",
to_char(sum(decode(substr(time,10,2),'18',1,0)),'99') "18",
to_char(sum(decode(substr(time,10,2),'19',1,0)),'99') "19",
to_char(sum(decode(substr(time,10,2),'20',1,0)),'99') "20",
to_char(sum(decode(substr(time,10,2),'21',1,0)),'99') "21",
to_char(sum(decode(substr(time,10,2),'22',1,0)),'99') "22",
to_char(sum(decode(substr(time,10,2),'23',1,0)),'99') "23"
from
v$log_history
group by
substr(time,1,5);
In sum, this v$log_history script is
great for showing the relative update activity of your database and it's one of
many tools used by the Oracle tuning professional.
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Tuning: The Definitive Reference", with 950 pages of tuning tips and
scripts.
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