Re: LOCALLY MANAGED EXTENT PERFORMANCE

On 4/26/05, Tim Gorman <tim_at_evdbt.com> wrote:
> > Assuming autoextention on the datafiles, why are you worrying about
> > how many extents until you run out of disk space? It seems to me that
> > it now makes more sense to keep on eye on disk space itself. Unless
> > you are trying to run your mountpoints to 99% full or you have small
> > mountpoints, ask the sysadmins to set a warning at X% and an alarm at
> > Y%.

>=20
> Autoextension can *postpone* the need for this kind of monitoring, but do=
es
> not eliminate it. Think it through.

My thinking goes something like this:
Autoextention allows the database to happily add extents, and hence grow files, at need. At some point this file growth will eat up my disk space. Therefore, I should monitor my disk space.

It appears to me that counting the number of extents able to be allocated and watching the mount point free space tries to achieve the same thing through different means. I also think that given the current technology mix of large mount points and Oracle's LMT and ATTM, monitoring mountpoints is the way to go. I think that because it is much easier to find a mountpoint's free space than it is to reverse-engineer ATTM's allocation algorithm (especially when Oracle can change it at will).

Further, let's take autoextention out of the picture. Even in a fixed-size tablespace with growing objects, I believe it is better to measure free space as a percentage instead of trying to count extents.  Counting extents will give you an exact number but a percent measurement, though approximate, is good enough for trend analysis.=20 That is, if you need the granularity provided by extents versus a percentage, you are already in a world of hurt.

Actually, percent free is fine but sometimes absolute numbers are just as good or preferred. "Let's see, 5GB growth/day and 20GB disk free.=20 I knew I forgot something for last quarter's budget."

> It's not a good idea to assume autoextension (or even extensibility) in a=
ll
> scenarios, whether at the datafile level or at the file-system level.

I'll agree with that. Lo many years ago I worked at a shop where disk space was always tight and mount points were very small and numerous.=20 We counted extents and had to respond to automated pages quickly in order to keep from running out of space. Ug.

However, I think in a strong majority of shops today disk free space monitoring is perfectly fine and much easier to do.

Finally, you said:
> Think it through.

I'd love to hear how you come to a different conclusion.

Steven

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