Mirroring Issue

[KOG]

Hi folks,

I am in the middle of configuring the oracle database on our new server F50 ! Now Oracle has its own issue about mirroring the datafiles using RAIDs. Aix have its own concepts - mirroring lv or fs or disk.

Now the issue here is if I have a filesystem that contains oracle datafile, should I only mirror the filesystem or should I mirror the logical volume that holds the filesystem?

What difference would that make, does it take up so much space? I have two 9gbs disks and one is already full as I have set two copies of LV, one on each disk.

Does anyone have any experience or knowledge of mirroring the oracle datafiles? Any feedbacks or suggestions.

Thanking you all in advance.

Regards

Katherine

 

[DSMARWAY]

If you are using RAID , itr depnds on how you have created your RAID set i..e normal mirroring , striped

The structure we normally follows is this :-

1. Install oracle code in a separate filesystem on a separated disk. Which will be mirrored at filesyetm level

2. Your redo logs , create them on a separate disk on a separate filesystem which is mirrored

3. If your dbase is in archive log mode then again use a separate disk and separate mirrored filesystem for this

4. Now your data files , if you have the disks you can spread the data files over these disks striped and mirrored
. These can be separate filesystems for each database.
( striping for better performance over more disks)

You can for , performance to create a raw logical volume which is mirrored , and using oracles async I/O to write direct to the raw LV's , rather than writing to filesystem first and then the logical volume .

So you probably have guessed , mirroring is a MUST , but depending on certain performance issues depends on how the data files are created i.e. striped,raw , just filesystems

What you need to take into consideration is if your database crashes , what files do you need to recover your database.

What I have listed above is just basic info and hope it gives you some idea .
Have you tried oracles metalink site ( requires username & password) they would have plenty of info on this subject and in detail.

 

[Chapter11]

Just a point: In AIX, you don't mirror filesystems, you mirror LV's.

 

[KOG]

Hi Chapter11

I am totally confused here now with the concepts of RAIDing the disks, Oracle gives wonderful documentation on RAIDing the disks, but the question here is how is that done at AIX level? When I ask Oracle how this is configured the answers I get is oh it depends on o/s and hardware you have !!! So that brings me back to square one.

As AIX only mirros the LVs so all the files within the Logical Volume is mirrored, what if there are some files or filesystems within that LV that I do not need to be mirrored, that would be waste of space.

What abt stripping, how is that achived at AIX level? I am just concern on how best to configure the disks for mirroring and stripping to ensure the space are used economically.

I wonder is there anyone out there who had this experience of mirroring or stripping oracle datafiles across several disks. At the moment, we have two 9gb disks for oracle VG but I now thing it is not sufficient to configure the files for RAIDing.

Thanking you all in advance.

Regards

Katherine

 

[DSMARWAY]

If you create a logical volume Fred which is 100MB , you cannot copy files to it by using normal cp , you can use something thing like dd , or let oracle do it thats if you tell it , my data file is going to be raw and it will be called fred. This will be 100MB in size whether you use all of it or not , because you have physically used this space.

If for some reason this LV fred is corrupted , you are stuffed becuase you only have one copy i.e. not mirrored
so you can mirror this LV fred normally on another disk . So you have to make sure you have 100MB free space on another disk.

If you create a filesystem using the LV fred called fredfs
you are basically adding a layer on top of the LV so you can
add more files into it , but you wil not have all the 100MB because depending on what type of filesystem you have created , it needs to know the number of inodes you need in the filesystem . So if you know the filesyetm will have alot of files in it you create it with a lot of inodes , and if you know there will only be a few files in it create it with less inodes ,hence you'll have more space.

As Chapter11 said , you mirror at the logical volume level
and depending on the size of the LV that can no longer be allocated ( unless you remove the LV)


So , if you create a filesystem , and now you want to create an oracle data file ( i.e. tablespace) , you tell oracle that you want to create a data file ( i.e. table space) in the filesystem you have created in our example the maximum can only be 100MB , you may want to create your data file which is 50MB and then at a later time increase the data file as required.

Does this make any sense ? I may be wrong but I think you may be getting confused with oracle data files ( table space and AIX logical volumes)

So it is good to mirror whether you have created a filesystem ontop of the LV or just using the LV on it own?

With AIX 4.3.3 you have the ability to stripe and mirror using IBM LVM . This is Physical extent based striping .
I think you can do it with 2 disks , but more disks you have the better it is , we have created a 5 disk mirrored stripe set ,which has oracle datafiles on it.

If you have Hardware RAID) , I think you need a minimum of 3 disks to achieve striping.

When ORACLE say it depends what H/w you have and OS , means only on AIX 4.3.3 you have the ability to do LV stripe + mirror , and whether you have a RAID card in your machine to allow you to perform hardware RAID .

does this make sense or are you still confused?

 

[Chapter11]

IMAO, Oracle should shut their trap about RAID. That always occurs at a layer well below them. Protecting LVs and filesystems is a function of the host OS, not an application.

There are several varieties of RAID. RAID1 is disk mirroring, although not all mirrors qualify to be called RAID1. AIX's software mirroring of LVs is one example I wouldn't call proper RAID1, although its fault tolerance (if properly configured) is the same.

If all you have is a pair of 9's, mirroring your LVs should be sufficient, although if that pair of disks are the only two members of the volume group, turn off quorum for that VG off so the VG itself can survive a 1-disk failure (requires the VG be varied off then back on to take effect).

 

[Yegolev]

Just FYI, Oracle says a lot of stuff but you should consider your environment when making decisions. Your machine is "small" so for you mirroring will be fine. If you mirror across two 9GB disks, your db cannot exceed 9GB and that is easily restored from 8mm tape, worst case. Personally I would suggest not thinking about striping until your db gets "large" and you feel you need the performance or redundancy gains. Our smaller systems have maybe ten or fifteen 7.2GB fs that contain one or more datafiles, but we only stripe the largest one (83 data fs) for performance reasons. IBM Certified -- AIX 4.3 Obfuscation

 

[bi]

Chapter11, I agree with you about Oracle's comments about RAID -- performance so much depends on what kind of hardware you have implemented RAIDx on. If it's old technology, you're going to get lousy results.

KOG, if you only have 2 9 GB disks available for your database, you are not going to be able to fully implement Oracle's OFA recommendations. Those recommendations mainly talk about mount points. What is buried somewhere in Oracle's rhetoric is that the mount points, for most efficient performance and data protection, need to be on separate disks and preferably the disks are going to be on separate controllers. And, of course, you have to watch where you are mirroring your LVs to.

This doesn't mean you are not going to be able to install the database and get it running. It just is not going to perform as well as it could with more disks and controllers to spread the data files across. I've just completed some performance tests and get at least 3 1/2 times better performance when I can spread everything across several disks and controllers.