ASM Environment multiplexed redo logs

When we have multi diskgroups like ('+DATA','+FLASH) and configured to multiplexed redologs on asm environment what would be the performance impact while logwriter writing on multiple locations? The diskgroups have different number disks on each diskgroup. Like 100 vs 10 disks on diskgroup.

http://download.oracle.com/docs/cd/B19306_01/server.102/b14231/onlineredo.htm
Multiplexing Redo Log Files help protect against a failure involving the redo log itself.
after you implement, you should check AWR report about waiting.
On ASM, you don't need to make multiple redo files. Just place it (redo file) in good i/o diskgroup (RAID1/0)

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Log_file_name_convert , db_recovery_area and multiplexed redo logs

Hi, I have a confusion here, need help.
I have multiplexed redo log members on both asm diskgroup DATA, FRA.
My log_file_name_convert='+DATA', '+DATA'
and db_recovery_area ='+FRA'
db_create_online_log_dest_1=+DAT
Are above parameters right?

log_file_name_convert is simply a string replacing function.Its same behaviour like DB_FILE_NAME_CONVERT
So all entries of <first> will be replaced by <second> and the <third> entry will be replaced by the <fourth> and so on.Yes the strings should be in even numbers like 2,4,6,8.. ther should be sequence for the source & destination diskgroup and so on.
So if you would have:
'DATA','REPLACE','FRA','REPFRA'
all occurences of DATA will be replaced with RAPLACE. But this would also mean ORADATAGRAM would be replaced to ORAREPLACEGRAM.If any ORL's / SRL's are created in DATA in source, by mentioning LOG_FILE_NAME_CONVERT those will be created to the connected proceeding string...
Can you point out please where is mistake in my previous post?

Multiplexing Redo Log Files question

If you are running RAC on ASM on a RAID system, is this required? We are using an HP autoraid which mirrors at the block level and in the documentation about Multiplexing Redo Log Files it says that you do it to protect against media failure. The autoraid that we are using gives us multiple levels of redundancy against media failure so I was wondering if Multiplexing would be adding more overhead than is needed. Thanks for your input.

ASM is quite compex and I'm not going to outline all the advantages or reasons for ASM, but under ASM you can drop and add devices to maintain your capacity needs online without loosing data, which you cannot do using RAID, which requires a re-initialize, for example, regardless of redundancy. Please see the documentation. ASM, like pretty much everything Oracle will add complexity and you will have to check your requirements. ASM is however pretty much the standard. If you use external RAID, make sure your storage is not using RAID 5 or 0. Regarding logical errors, you could for example overwrite or delete a file by mistake, in which case file redundancy does not protect you. If you are looking for reasons or ways not to use ASM, I'm sure you will find them, but what's the point?

Multiplexing redo log files

I am using 9i R2 database.
I just want to clear some doubts.
I am having 3 redo log groups with one member each.
Now for mutiplexing redo log files
i want to add one one member to each group so
that each group will have 2 members in some different path.
my current path of log member are
/oracle/files/a1.log (group 1)
/oracle/files/b1.log (group 2)
/oracle/files/c1.log (group 3)
I want to add
/oracle/sysfiles/a2.log (group 1)
/oracle/sysfiles/b2.log (group 2)
/oracle/sysfiles/c2.log (group 3)
A--So is there any need to take database in any mode (nomunt,mount) or i can do it while
database is open ?
I will issue
ALTER DATABASE ADD LOGFILE MEMBER /oracle/sysfiles/a2.log TO GROUP 1 ;
First in inactive group,then
alter system switch logfile; (To make 2 group inactive....and then 3)
B-- All memebers in a redolog group must be of same size? if i am having
100 M.B. of one member(a1.log) is there no need to specify size while adding new member(a2.log)?
Can some one validate and correct my steps or suggest me regarding this?

Hi,
All the solutions are in oracle documentaiton.
See, Members of the same multiplexed redo log group must be the same size (that all member in with in the group ) and Members of different groups can have different sizes. If the same of Files are different resultant would be the with Checkpoints. Since you can not gurantee that Checkpoints will occur at regular internals (Log switch with respect log members). Better to follow the Rules of traffic similarly follow the instrcutions of ORACLE Documentation.. :-)
- Pavan Kumar N

Multiplexing Redo Log and maximum protection mode.

Assume that during writing into redo logs the instance crashes. As a result, members of active redo group are not synchronized, some of them had more data. How Oracle will handle this when instance starts? And there can be case when at startup time some members that had more redo before crash, are lost.
Now assume that we have standby database with maximum protection mode. After LGWR has written to local redo logs and before writing to standby redo logs, primary instance crashes. In this case standby site lost last transaction.
Is it correct? Thanks.

Assume that during writing into redo logs the instance crashes. As a result, members of active redo group are not synchronized, some of them had more data. How Oracle will handle this when instance starts? And there can be case when at startup time some members that had more redo before crash, are lost.
Members of a particular group are written concurrently by LGWR, all members of a log group will have same data. If any member of a particular group is lost or not reachable, oracle will read from the available log member during instance recovery.
Multiplexing Redo Log Files
http://docs.oracle.com/cd/B19306_01/server.102/b14231/onlineredo.htm#i1006249
To answer your second question,In this mode no transaction commits on primary unless the redo is also written on atleast one standby database, otherwise primary will go down.
Check below
Maximum Protection
http://docs.oracle.com/cd/B28359_01/server.111/b28294/protection.htm#CHDHFHJI

Why multiplex redo log group ?

Hello,
Why should we multiplex redo log groups if we have only file system which is already mirrored. Is there any one who had an incident when he has only one redo log member per group placed on a mirrored file system but still got corrupted and he felt better to have multiple members even if they reside on the same file system (which is mirrored)
Thanks
Salman

Ansiktet wrote:
EdStevens wrote:
Salman Qureshi wrote:
Hello,
Why should we multiplex redo log groups if we have only file system which is already mirrored. Is there any one who had an incident when he has only one redo log member per group placed on a mirrored file system but still got corrupted and he felt better to have multiple members even if they reside on the same file system (which is mirrored)
Thanks
SalmanThe mirror won't protect you from an SA who deletes '/u01/oradata/redo01.log' because he is running out of space on /u01 and figures its safe to delete a log file.
Or similar types of errors.
The redo and control files are simply too critical to put all your eggs in one basket when planning their protection.:) Thats why you should not use Oracle default .log on redo, instead .dbf or .dbl can be used. THat's why I use the older (pre-10g) default of .rdo for redo logs
However, has this happend anyone for real, that a SA delete Oracle files? Where do you think I came up with the example?
HOw about an SA (or maybe the kind of "fresher" we often see here, Taking a look at a "log" file with notepad?
If he is isnt stupid he should know that Oracle resides on /u01 partion (for example), and should not delete files there without consulting the DBA."should" is the operative word there. There is no accounting for corporate cultures and attitudes.
But end the end, my example was to illustrate that not all problems with redo and control files are mitigated with disk mirroring.

Multiplexing redo logs and control files to a separate diskgroup

General question this one...
I've been using ASM for a few years now and have always installed a new system with 3 diskgroups
+DATA - for datafiles, control files, redo logs
+FRA - for achive logs, flash recovery. RMAN backup
Those I guess are the standards, but I've always created an extra (very small) diskgroup, called +ONLINE where I keep multiplexed copies of the redo logs and control files.
My reasoning behind this is that if there are any issues with the +DATA diskgroup, the redo logs and control files can still be accessed.
In the olden days (all those 5 years ago!), on local storage, this was important, but is it still important now? With all the striping and mirroring going on (both at ASM and RAID level), am I just being overtly paranoid? Does this additional +ONLINE diskgroup actually hamper performance? (with dual write overheads that are not necessary)
Thoughts?

Some of the decision will probably depend on your specific environment's data activity, volume, and throughput.
Something to remember is that redo logs are sequential write, which benefit from a lower RAID overhead (RAID-10, 2 writes per IOP vs RAID-5, 4 writes per IOP). RAID-10 is often not cost-effective for the data portion of a database. If your database is OLTP with a high volume of random reads/writes, you're potentially hurting redo throughput by creating contention on the disks sharing data and redo. Again, that depends entirely on what you're seeing in terms of wait events. A low volume database would probably not experience any noticeable degraded performance.
In my environment, I have RAID-5 and RAID-10 available, and since the RAID-10 requirement from a capacity perspective for redo is very low, it makes sense to create 2 diskgroups for online redo, separate from DATA, and separate from each other. This way, we don't need to be concerned with DATA transactions impacting REDO performance, and vice versa, and we still maintain redo redundancy.
In my opinion, you can't be too paranoid. :)
Good luck!
K

Multiplex Redo Logs and Control File

I am wanting to setup an existing Oracle Express 10g instance to multiplex the redo log files and the control file.
Instance is using Oracle-Managed Files and the Flash Recovery Area.
With these options being used what are the steps required to setup multiplexing?
I tried setting the DB_CREATE_ONLINE_LOG_DEST_1 and DB_CREATE_ONLINE_LOG_DEST_2 parameters but this doesn't appear to have worked (I even bounced the db instance).
BTW, the DB_CREATE_FILE_DEST is set to null and the DB_RECOVERY_FILE_DEST is set to the flash recovery area.
Any help is much appreciated.
Regards, Sheila

Thanks for this. My instance originally had two log groups so I've added a new member to each group into the same flash recovery area directory, but have assigned a name. Is this why when I query v$logfile the is_recovery_dest_file is set to NO? Is it ok to assign a name & directory and if not, how do you add a new memeber and allow Oracle-Managed files to name them?
Also, how can I check that the multiplexing is working (ie the database is writing to both sets of files)?
Thanks again.

ASM Migrate Archive Redo Logs

Hi all,
Is there any way to migrate existing archived redo logs to the ASM diskgroup?
Regards,
Vijayaraghavan K

Thanks..
I have copied the spfile from the last backup to the asmdiskgroup by using below command
RESTORE SPFILE TO '+DATA/spfilesid.ora';
When i try to start the database it is picking the spfile from file system alone. It is possible to rmap the spfile to the diskgroup?
Regards,
Vijayarghavan K

What will heppen if redo logs at os level get deleted

Friends,
need 1 answer about the query: what will heppen if redo logs at os level get deleted.

how can i find where are my multiplexed redo logs, so that i can copy it from there
SQL> shutdown abort
ORACLE instance shut down.
SQL> startup mount
ORACLE instance started.
Total System Global Area 534462464 bytes
Fixed Size                  2256912 bytes
Variable Size             398462960 bytes
Database Buffers          125829120 bytes
Redo Buffers                7913472 bytes
Database mounted.
SQL> select * from v$logfile
2 ;
    GROUP# STATUS TYPE
MEMBER
IS_
         3         ONLINE
C:\O\ORADATA\XE\REDO03.LOG
NO
         2         ONLINE
C:\O\ORADATA\XE\REDO02.LOG
NO
    GROUP# STATUS TYPE
MEMBER
IS_
         1         ONLINE
C:\O\ORADATA\XE\REDO01.LOG
NO                                         However, Oracle knows about the multiplexing, so if your logs were multiplexed properly (using redo log members), Oracle would already have known about them and identified that there was one available.
Just goes to show you (if this is, in fact what you did), don't just go blindly deleting files that have a ".LOG" extension.

How does LGWR write redo log files, I am puzzled!

The document says:
The LGWR concurrently writes the same information to all online redo log files in a group.
my undestandint of the sentence is following for example
group a includes file(a1, a2)
group b includes file(b1, b2)
LGWR write file sequence: write a1, a2 concurrently; afterwards write b1, b2 concurrently.
my question is following:
1、 my understanding is right?
2、 if my understanding is right, I think that separate log file in a group should save in different disk. if not, it cann't guarantee correctly recovery.
my opinion is right?
thanks everyone!

Hi,
>>That is multplexing...you should always have members of a log file in more than 1 disk
Exactly. You can keep multiple copies of the online redo log file to safeguard against damage to these files. When multiplexing online redo log files, LGWR concurrently writes the same redo log information to multiple identical online redo log files, thereby eliminating a single point of redo log failure. In addition, when multiplexing redo log files, it is preferable to keep the members of a group on different disks, so that one disk failure will not affect the continuing operation of the database. If LGWR can write to at least one member of the group, database operation proceeds as normal.
Cheers
Legatti

Quick Redo Log Question:

Hi All,
I am attempting to multiplex redo logs with Enterprise Manager and there is an option to 'reuse file'. I do not see any explanation about what this option does in my manuals and would appreciate if someone could quickly explain the what and why I would want to select this.
Thanks in advance,
d.

Hi All,
I am attempting to multiplex redo logs with
Enterprise Manager and there is an option to 'reuse
file'. I do not see any explanation about what this
option does in my manuals and would appreciate if
someone could quickly explain the what and why I
would want to select this.
Thanks in advance,
d.The REUSE option is discussed in the SQL Reference manual under the description of the ADD [STANDBY] LOGFILE MEMBER Clause. You need to use this option if you wish to use existing file(s) for the new multiplexed log file members in the group.

Multiplex Redo logfiles

Hi,
I have a system that has very high I/O load and we run 35 databases on the same server.
All databases is configured with Multiplex redologs and I was considering removing multiplex to help speed up the system and offload the I/O on the system.
I know there are a risk not running multiplex redo logs but is this mainly due to if someone deletes the files or are we talking about possible corruption of the redo logs and loss of data?
As I've never done it before i have to ask:
If I delete the active redolog will the instance stop and become corrupt?
Regards
Morten

Rob_J wrote:
You have good points, EdStevens, but my company are not prepared to take the performance hit. I know that someone could delete them at the OS level, but our SAs know not to touch anything to do with the DBs before asking....If performance is so important than why not to put the redo log files over SSD? The point that Ed has made are very relevant. A hardware RAID won't protect you from human mistakes and this is just a matter of time when you see that a SA is going to come and mention that he didn't know that it was a set of files needed for the database.
If your DBs are log switching rapidly this will add more I/O because of the checkpointing process which has to happen each time there is a log switch. If you have a very large number of small datafiles in your DB, this will increase the time taken to update all the datafile headers, also.The much better way to overcome is this to make VERY large redo logs and control the switching of them using the parameter archive_lag_target parameter. This would solve your issues of keep fiddling the size of the redo logs in varying workloads.
If your DBs are DEV DBs and not PROD ones, and you can restore/refresh them easily, I'd go with one member per log group.
In our DEV DBs we don't run them in archivelog mode either, because we aren't too worried if we lost the DB; we can recreate it easily enough that the positives of having it in archivelog mode don't outweigh the negatives.
If you have optimised everything else, and you are still maxing out the I/O, perhaps you can ask for a hardware upgrade. Also remember that you might be able to tune the statements being run within the DB and look at resource manager to share out resources more evenly.I shall just say that its much easier said than done that you would get a hardware upgrade just like that and also that you would be able to find a db where everything is optimized.
Just my 2 cents.
Aman....

REDO Log Members

Hello-
I was looking at some old database installs and if the REDO logs are stored in the same file directory then isn't having two members
not very useful and only good for placing these on another file system ?
Thanks-

Admin. Guide says http://download.oracle.com/docs/cd/E11882_01/server.112/e17120/onlineredo002.htm#ADMIN11309
>
To protect against a failure involving the redo log itself, Oracle Database allows a multiplexed redo log, meaning that two or more identical copies of the redo log can be automatically maintained in separate locations. For the most benefit, these locations should be on separate disks. Even if all copies of the redo log are on the same disk, however, the redundancy can help protect against I/O errors, file corruption, and so on.

Best way to move redo log from one disk group to another in ASM?

Hi All,
Our db is 10.2.0.3 RAC db. And database servers are window 2003 server.
We need to move more than 50 redo logs (some are regular and some are standby) which are not redundant from one disk group to another. Say we need to move from disk group 1 to 2. Here are the options we are thinking about but not sure which one is the best from easiness and safety prospective.
Thank you very much for your help in advance.
Shirley
Option 1:
1)     shutdown immediate
2)     copy log files from disk group 1 to disk group2 using RMAN (need to research on this)
3)     startup mount
4)     alter database rename file ….
5)     Open database open
6)     delete the redo files from disk group 1 in ASM (how?)
Option 2:
1)     create a set of redo log groups in disk group 2
2)     drop the redo log groups in disk group 1 when they are inactive and have been archived
3)     delete the redo files associated with those dropped groups from disk group 1 (how?) (According to Oracle menu: when you drop the redo log group the operating system files are not deleted and you need to manually delete those files)
Option 3:
1)     create a set of redo members in disk group 2 for each redo log group in disk group 1
2)     drop the redo log memebers in disk group 1
3)     delete the redo files from disk group 1 associated with the dropped members

Absolutely not, they are not even remotely similar concepts.
OMF: Oracle Managed Files. It is an RDMBS feature, no matter what your storage technology is, Oracle will take care of file naming and location, you only have to define the size of a file, and in the case of a tablespace on an OMF DB Configuration you only need to issue a command similar to this:
CREATE TABLESPACE <TSName>; So the OMF environment creates an autoextensible datafile at the predefined location with 100M by default as its initial size.
On ASM it should only be required to specify '+DGroupName' as the datafile or redo log file argument so it can be fully managed by ASM.
EMC. http://www.emc.com No further commens on it.
~ Madrid
http://hrivera99.blogspot.com

ASM Environment multiplexed redo logs

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