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Physical Vs logical Volume

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mehul_3
Regular Advisor

Physical Vs logical Volume


What is the basic difference between logical and physical volume.
As per oracle recommendation, physical files are segregated on different disk drive if hot spot occurred. But I am always in doubt of whether segregation should be on logical or physical volume basis. Our database’s datafiles is separated on logical volume basis and when I issue the command like sar –d to find which physical volume is more %busy then only one physical volume say c4t0d2 shows me more busy and another c4t0d0 is very less.
Can someone shed some light on the issue

Thanks
Mehul


6 REPLIES
Pete Randall
Outstanding Contributor

Re: Physical Vs logical Volume

A logical volume is an LVM concept wherein one or more physical volumes can be divided up into logical volumes. A physical volume may be broken up into several logical volumes or several physical volumes be grouped together into one large logical volume.


Pete

Pete
Kent Ostby
Honored Contributor

Re: Physical Vs logical Volume

Mehul --

Physical volumes are essentially disk drives or LUNS.

A logical volume can be a part of a disk or multiple disks group together.

Physical == the actual hardware you are using.

Logical volume == the way that you want to partition that physical space to be used.
"Well, actually, she is a rocket scientist" -- Steve Martin in "Roxanne"
Stuart Abramson_2
Honored Contributor

Re: Physical Vs logical Volume

Try running this script on your system.

It will show your logical volumes as they sit on the physical volumes (inside the Volume Groups).

There is probably some documentation that you can read to explain this (on the ITRC) if you do some kind of document search.
Howard Marshall
Regular Advisor
Solution

Re: Physical Vs logical Volume

I don't know if this is something you already know or what but the difference is really not that complicated.

A physical volume is just a disk drive, something you can put in your hand. The Computer can read and write data to it, that data can be a file system or a raw space for database, paging space, raw data dumping, or what ever else you do with a disk drive. A logical volume is a set of partitions on one or many physical volumes that the system treats just like it were a separate physical volume.

The LVM system is a layer between the computer and the disks. It takes control of any physical volumes or disks that you assign to it and effectively splits the disk up into as many 4 meg partitions as it can. Then it strings together a bunch of those partitions and creates a logical volume or "imaginary" physical volume that the computer can treat as a separate disk drive.

The benefits are obvious, you have a bunch of these partitions, any one of which can be part of a logical volume. If one of your logical volumes has a file system that gets too full, you can tack on a few more partitions (extend the lv) and make the file system a little bigger (you can even do it run time if you have jfs)

So, back to your issue. If all of your logical volumes for oracle are using the same physical volume, then there is no load balancing that can be done sense the logical volumes it could spread the load over are all coming from the same device. If, however, you have several physical volumes or disks in you volume group you can either spread the logical volumes across them, so that the 4 meg partition string takes one from this disk, the next from another, the next from a third, then the next from the first disk and so on. That will help to load balance the disks behind the oracle scenes. The drawback to that is if you loose one disk you loose a little bit of every logical volume. The other option is to make several logical volumes on each disk then let oracle load balance their use for you. Then if you loose a disk, you only have to restore the databases that were on those logical volumes instead of the entire database.

Hope that helps some.

H
Jean-Luc Oudart
Honored Contributor

Re: Physical Vs logical Volume

Mehul

from HP3KOracle presentation :
"The example (from HP/Oracle IT internal training) suggests features such as having multiple disk mount points so that disk I/O can be isolated. It also recommends mirroring redo logs, control files, and other database files. These are great ideas in general, but implementing these features in the current HPUX environment only decreases performance and provides very little additional reliability. The disk subsystems on HPUX today usually take care of disk mirroring on a hardware level. Distributing data files to independent disk spindles actually yields worse performance than putting all data files on a single volume that is distributed-striped across at least four drives. We contend that our solution, the Optimal Simplified Architecture for Oracle on UNIX, is more compliant to the OFA standard than the example given in Oracle training.
"
follow this link :
http://h21007.www2.hp.com/dspp/files/unprotected/database/HP3KOracle.ppt

Regards
Jean-Luc
fiat lux
podichin
Advisor

Re: Physical Vs logical Volume

Hi Mehul..,

Better option is first you check what are logical volumes on your most his physical volume.,

U can check it with
pvdisplay -v /dev/dsk/c4t0d0 command, This coomand will show u the lvols inside this physical volume, And then check the mount point for this lvols,
Ex:

Can check it wih bdf command..,

Then check your datafile and table space status.,

Like what are all the datafiles inside this mount point and which table space is using this datafiles, And if this mount point contains more datafiles and hit ratio is very high..,

U need to think about moving it some other disks.,

Cheers..,

Thanks and regards,

Siju