Re: AU size for multi-TB DB

The AU size in ASM is set per ASM diskgroup. For your 100 tb system, something like 4 mb should be sufficient. I have tested in a fiber channel environment and did not notice a significant difference going higher than 4 mb. For a 700 tb size, I would start a bit higher, at least 8 to 16 mb.  In a Dataguard environment, try to make sure all systems have the same AU. It will work with different values, but there is a chance you can run into unexplained performance issues and the targets fall behind. This is most noticeable with high rates of changes for LOBs and Secure files, where a DG related query starts doing a full table scan into ASM. If you are doing a migration using DG then you may have no choice if the old source system is not at the AU you desire. Also try to get the ASM compatibility levels as high as possible, because these have an effect on performance as well. My workaround for the really odd DG-LOB apply lag was to update the ASM compat levels to 19.0.0.0. As far as I know, there is no easy way to change an existing AU setting, other than to copy all objects to a new diskgroup. Lastly, remember the tablespace limit is dependent on the block size of the diskgroup and the db_cache. To get above 32 tb you need to add higher block sizes. 8k=32tb limit up to 32k=128tb. I have not checked if this has changed in 21c+. Kind regards
Jon Crisler

On Thu, Mar 28, 2024 at 10:04 AM Stefan Koehler <contact_at_soocs.de> wrote:

> Hello Laurentiu,
> are you still using Oracle 10g? If not, Oracle ASM uses variable size
> extents, e.g. check here:
> https://docs.oracle.com/en/database/oracle/oracle-database/19/ostmg/asm-intro.html#GUID-1E5C4FAD-087F-4598-B959-E66670804C4F
>
> "Each extent resides on an individual disk. Extents consist of one or more
> allocation units (AU). To accommodate increasingly larger files, Oracle ASM
> uses variable size extents. Variable size extents enable support for larger
> Oracle ASM data files, reduce SGA memory requirements for very large
> databases, and improve performance for file create and open operations. The
> initial extent size equals the disk group allocation unit size and it
> increases by a factor of 4 or 16 at predefined thresholds. The various
> extent sizes are described in this topic.
> ...
> ..."
>
> Best Regards
> Stefan Koehler
>
> Independent Oracle performance consultant and researcher
> Website: www.soocs.de
> Twitter: _at_OracleSK
>
> > Laurentiu Oprea <laurentiu.oprea06_at_gmail.com> hat am 28.03.2024 14:08
> CET geschrieben:
> >
> >
> > Dear oracle community,
> >
> >
> > Did anyone followed and tested recommendations from oracle note:
> Deployment of very large databases (10TB to PB range) with Automatic
> Storage Management (ASM) (Doc ID 368055.1)
> > when deployed a multi-TB database (in my scenario 100+TB and 700+TB) ?
> >
> > If yes and run some tests I appreciate if you can share some results /
> conclusions
> >
> > Thank you
> --
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>
>
>

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