LGWR will always write to the redo-logs in a "round-robin"
fashion. So, on completing Group2, it would wait
for Group3 to be INACTIVE. {One exception is when you ADD a new Log
Group. If you added a new log
file without specifying a Group#, it would be allocated Group#4, would be
set to status UNUSED and would
get used before Group3 if LGWR is still currently in Group2}
In terms of disk space "many smaller log groups" is the same
requirement as "few large log group".
Larger log groups are preferred by many DBAs so as to reduce the
frequency of the Log Switch Checkpoint.
{provided, of course, that LOG_CHECKPOINT_INTERVAL is set to higher than
the size of the log files}.
In scenarios where you have Standby Database and wish to propagate
Archive Logs frequently to
the Standby Site, you would have to accept frequent Log Switches
{however, this does not mean that
the Log Files have to be small, they could still be large}
Hemant
At 03:59 PM 30-01-04 -0800, you wrote:
Thanks Mladen and David
It is great to get such detailed info.
If we take your example of three redo logs then you could have a
situation where one is is 'CURRENT' status and two are in 'ACTIVE'
status. As follows :
GROUP# STATUS
---------- ----------------
1 ACTIVE
2 CURRENT
3 ACTIVE
Now if group 2 ('CURRENT') fills up or if 'alter system
switch logfile' command is given then it will try to go to the next one.
Since the next one (in fact both) is 'ACTIVE' it cannot be allocated and
made current. Right? So in that case will the database hang? Will it wait
for the next sequence redo log which is group 3 to get 'INACTIVE' or will
it allocate
group 1 if it becomes 'INACTIVE' before group 3.
Basically given limited space is it advisable to have many smaller redo
log groups instead of Oracle suggested bigger redo logs which switch once
an hour.
I agree it is unlikely and might as well just get extra space but the
point is to put in the best possible setup and be ready for crazy
activity instead of just going by the 'thumb rule' of having one log
switch per hour.
Thank you again
Mladen Gogala <mladen@wangtrading.com> wrote:
- David Hau explained this much better and in much more clear
- fashion then me. Here is the most important part from the usenet
- thread he was referring you to:
- **********************************************************************
- This makes sense if you think about where the various v$ dynamic
- performance
- views get their info from, and which Oracle background process is
- responsible for each task. First, note that:
- 1. v$log.status gets its redo log info from the *control file*
- 2. v$datafile_header.checkpoint_change# and checkpoint_time get their
- info
- from the *datafile headers*.
- Here's the sequence of events when a log switch happens:
- 1. LGWR switches to the next redo log file, changes the status of
the
- previous redo log file from CURRENT to ACTIVE in the control file,
and
- signals DBWR to do a checkpoint on the previous ! redo log file.
- 2. When DBWR finishes with the checkpoint, it signals CKPT to
update
- datafile headers and update checkpoint info (only) in the control
file.
- This is the info read by v$datafile_header.checkpoint_change# and
- checkpoint_time. Note that CKPT does not update redo log info in
the
- control file. It only deals with checkpoint info, as its name
implies.
- 3. When CKPT is done, it signals LGWR to update the redo log status
in
- the
- control file from ACTIVE to INACTIVE. This is the info read by
- v$log.status. This update task is a low priority item for LGWR
because
- the
- only process that cares about whether the redo log status is active
or
- not
- is LGWR itself. The redo log status tells LGWR whether it can reuse a
- redo
- log file or not (i.e. whether checkpoint has completed on that redo
log
- file.) That is, by delaying this operation, LGWR is not blocking the
- work
- of any other process.
- LGWR will update the redo log status i! n the control file when any
of
- these
- occurs (and others too, that I don't know of):
- 1. when LGWR periodically checks for compliance with the
- LOG_CHECKPOINT_TIMEOUT parameter, which says that the checkpoint
- position
- should not lag behind the latest redo record by this amount of
time.
- 2. when you issue a "alter system checkpoint" which is what
you did.
- So if you want the redo log status to be updated more quickly to
- inactive
- after a checkpoint, one way to do it is to decrease the value of
- LOG_CHECKPOINT_TIMEOUT in init.ora.
- Cheers,
- Dave
- **************************************************************************
- On 01/30/2004 03:44:29 PM, A Joshi wrote:
- > Mladen,
- > Thanks for info. So all the dirty blocks need to be written to
disk
- > after each checkpoint. After that is done the status becomes
- > 'INACTIVE'. Just that sometimes this is very unpredictable.
- > My question : If a log switch alway! s causes a implicit
checkpoint
- > then
- > what is the need for this explicit checkpoint to be given? Thank
you.
- >
- > Mladen Gogala wrote:
- > On 01/30/2004 01:24:26 PM, A Joshi wrote:
- > > Hi,
- > > In view v$log there is a column status. This changes from
current
- > > (if the redo log is in use) to atcive then to inactive.
- > Documentation
- > > says :
- > >
- > >
- > > ACTIVE: The log is active but is not the current log. It is
needed
- > > for
- > > crash recovery. It may be in use for block recovery. It
might or
- > > might
- > > not be archived.
- > >
- > > INACTIVE: The log is no longer needed for instance
recovery. It may
- > > be
- > > in use for media recovery. It might or might not be
archived.
- > >
- > > So if it is in 'ACTIVE' status : it means it is needed
for
- > > crash/instance recover! y? So what does it mean? That
changes in this
- > > log are still not written to data files? What else? Is this
related
- > > to
- > > delayed block cleanout etc or no connection?
- > >
- > > What factor affects how long it will be in ACTIVE state
before
- > going
- > > to INACTIVE? What can be changed to control how long it
takes. Any
- > > trade off?
- > >
- > > Any detailed explanation on this will be greatly
appreciated. You
- > can
- > > mail me direct or to the list. Thank you.
- > >
- > Status of active means that log has been switched, but not all
of the
- > changes are written to the disk. It is, essentially, saying that
DBWR
- > has some more work to do to catch up. The only log that is open
by
- > the
- > instance is the one marked "CURRENT". Here is an
example:
- > SQL> select group#,status from v$log;
- >
- > GROUP# STATUS
- > ---------- ----------------
- > 1 CURRENT
- > 2 INACTIVE
- > 3 INACTIVE
- >
- > SQL> alter system switch logfile;
- >
- > System altered.
- >
- > SQL> select group#,status from v$log;
- >
- > GROUP# STATUS
- > ---------- ----------------
- > 1 ACTIVE
- > 2 CURRENT
- > 3 INACTIVE
- >
- > So, after the checkpoint, the CKPT has updated log file
headers,
- > corresponding markers and termination records are written to the
- > file,
- > data file headers have been updated and so has been the control
file.
- > DBWR still has some work to do because there are still
unwritten
- > blocks modified by transactions whose log records are in the
"ACTIVE"
- > file. DBWR will take its time to write them down.
- > The only log file open by the instance is redo02:
- >
- > [root@tux925 root]# fuser
/data/db/OraHome1/oradata/compldb/*.log
- > /data/db/OraHome1/oradata/compldb/redo02.lo! g: 6019
- > [root@tux925 root]#
- >
- > After another checkpoint, everyhing is back to normal:
- >
- > SQL> alter system checkpoint
- > 2 /
- >
- > System altered.
- >
- > SQL> select group#,status from v$log;
- >
- > GROUP# STATUS
- > ---------- ----------------
- > 1 INACTIVE
- > 2 CURRENT
- > 3 INACTIVE
- >
- > --
- > Please see the official ORACLE-L FAQ:
http://www.orafaq.net
- > --
- > Author: Mladen Gogala
- > INET: mladen@wangtrading.com
- >
- > Fat City Network Services -- 858-538-5051
http://www.fatcity.com
- > San Diego, California -- Mailing list and web hosting services
- >
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- > to: ListGuru@fatcity.com (note EXACT spelling of 'ListGuru') and
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- > the message BODY, include a line containing: UNSUB ORACLE-L
- > (or the name of mailing list you want to be removed from). You
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- > also send the HELP command for other information (like
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- --
- Please see the official ORACLE-L FAQ:
http://www.orafaq.net
- --
- Author: Mladen Gogala
- INET: mladen@wangtrading.com
- Fat City Network Services -- 858-538-5051
http://www.fatcity.com
- San Diego, California -- Mailing list and web hosting services
- ---------------------------------------------------------------------
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it!
Hemant K Chitale
Oracle 9i Database Administrator Certified Professional
http://hkchital.tripod.com
{last updated 24-Jan-04}
--
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INET: hkchital@singnet.com.sg
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Received on Fri Jan 30 2004 - 21:04:26 CST
