Re: What does EXTERN, NORMAL, HIGH types mean for ASM diskgroups?

JL wrote:

> Hi:
>
> We are not sure about what EXTERN, NORMAL, and HIGH
> mean for ASM Diskgroup types.
>
> For example, we have set up an ASM diskgroup with type HIGH.
> Our understanding is that HIGH means there are three copies
> of any file (i.e., there are two mirrors).

Not "file". But "piece of data". Your ASM diskgroup already consists of X hard disk partitions. All HIGH means is that Oracle will write a piece of data to three locations within the disk group.

> But, when Oracle starts writing out archived redo logs to ASM,
> why is there only one archived redo log written out?
> Should there not be multiple copies?

Only if you configure multiple archive destinations in your spfile. Until then, the one archive destination gets written to once, and ASM then simply makes sure that write is internally replicated to two other locations.

> If there is mirroring, where are the mirrored copies?
>
> Lastly, could someone please post the official definitions of EXTERN,
> NORMAL, and HIGH for ASM diskgroups?

ASM is not RAID. It's redundancy comes from writing data multiple times, not writing to multiple files on multiple disks. Of course, doing that only truly makes sense when the multiple writes are performed on multiple disks, but that's almost incidental given the question you're asking.

Think 'extents' or 'blocks', if that helps: if EMP is stored in HIGH redundancy mode, and I change KING's salary, when the block containing KING's revised record is flushed to disk, it will be flushed to three different locations within the one disk group.

That's my understanding of it, anyway. Official definitions of things are always available by searching the documentation at http://tahiti.oracle.com.

Regards
HJR Received on Wed Oct 20 2004 - 16:07:27 CDT