Re: Algorithm for calculating extent size in LMT

Utter nonsense, I'm afraid. You seem to be assuming that congruity of extents from the logical perspective translates into physical congruity. It doesn't. Even a single extent is comprised of physical o/s blocks scattered all round the disk platter, so the additional latency induced by not having extents 'next to each other' is barely detectable, if ever.

The number of extents has absolutely zero impact on the performance of full table scans or on bulk deletes (with the sole proviso that having to read the extent map itself in multiple i/o operations *is* a slight performance overhead).

HJR

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"dkrpata1" <dkrpata1_at_cox.net> wrote in message
news:14Gg8.26219$FY5.1048917_at_news2.west.cox.net...


> You're right when you cause all those extents to fragment like that

> continuously.  It's a single move for the disk controller to go to the

disk


> to get that info.  But what if you had 100s of tables with 100s of extents

> in the same datafile spread out.

>

> Ex. tables a, b and c

>

> extent map

> AAABBBCCCABACCABCABACCABCABCCABBC

>

> Then you're going to see the time increase in this scenario because of the

> jumping around of the I/O to get the info for that table from disk.

>

> But Mr Morgan, the only thing I'd have to say to your statement

>

> "And to me a large number of exents is a sign that someone doesn't

> understand their schema or didn't take the time to calculate the size of a

> table and its data ... in short ... sloppy and unprofessional

workmanship."


>

> Is that there are quite a few scenarios of DBA's creating, from scratch, a

> new database and sized it according to the functional specs only to find

out


> that the "functional" people, don't really function.  But in most other

> occasions I would agree with your statement.

>

> Dan Krpata

>

>

> "Howard J. Rogers" <dba_at_hjrdba.com> wrote in message

> news:a5ubp7$3lv$1_at_lust.ihug.co.nz...

> > You'd have to explain why a delete would take longer for a segment

> comprised

> > of (say) 100 1M extents than for a segment comprised of 10 10M extents.

> >

> > As a quick test, I created a tablespace with uniform size 256K.  I

created


> a

> > table, BLAH, as a select * from dba_objects.  I then 'inserted into blah

> > select * from blah' multiple times until I had 239 extents (and half a

> > million records).  Set timing on, and a 'delete from blah' took 59.09

> > seconds.

> >

> > Bounce the Instance, repeat all the above procedures, only this time

> create

> > the table in a tablespace with 2M extents, meaning that BLAH ended up

with


> > half a million records as before, but this time in only 29 extents.  Set

> > timing on, delete from blah: 1 minute and 1.04 seconds.

> >

> > The difference is not significant, and I wouldn't expect it to be.  The

> > number of extents has practically nothing to do with the work involved

in


> > deleting records.  And when the number of extents is more or less

> irrelevant

> > to such matters, not spending time worrying about them is not a sign of

> > sloppiness or unprofessionalism, but of sensible prioritising of the

DBAs


> > time.

> >

> > HJR

> > --

> > ----------------------------------------------

> > Resources for Oracle: http://www.hjrdba.com

> > ===============================

> >

> >

> > "damorgan" <dan.morgan_at_ci.seattle.wa.us> wrote in message

> > news:3C82AC99.4BFD214B_at_ci.seattle.wa.us...

> > > Thanks for adding the clarification to my posting.

> > >

> > > But I still see performance problems with large numbers of extents.

> > Especially

> > > when doing things such as emptying a table with DELETE rather than

> > TRUNCATE

> > > (something that is sometimes necessary). And to me a large number of

> > exents is a

> > > sign that someone doesn't understand their schema or didn't take the

> time

> > to

> > > calculate the size of a table and its data ... in short ... sloppy and

> > > unprofessional workmanship.

> > >

> > > Daniel Morgan

> > >

> > >

> > >

> > > "Howard J. Rogers" wrote:

> > >

> > > > There *is* such a thing, of course... there is an 'autoallocate'

> policy

> > for

> > > > locally managed tablespaces, and as best I can tell it goes

something


> > like

> > > > this:

> > > >

> > > > The first 16 extents of a segment will be 64K in size.

> > > > The next 64 extents will be 1M in size

> > > > Then extents become 8M in size.

> > > > At the 200th extent, you get 64M extents.

> > > > After that, I can't tell you... because I ran out of disk space!

> > > >

> > > > What Daniel is hinting at, I guess, is that having odd-sized extents

> > within

> > > > a tablespace is not a good idea, because it risks fragmentation.  I

> > agree

> > > > with him that 'autoallocate' is not a terribly good idea for your

own


> > > > tablespaces, and that you should take charge of the extent

allocation


> > > > policy.

> > > >

> > > > The essential feature of locally managed tablespace is that we no

> longer

> > > > really give a damn how many extents a segment acquires, because

extent


> > > > allocation is now a trivial operation for the database (though I

agree


> > that

> > > > having the extent map for a segment fit into one block makes for

some


> > small

> > > > performance improvement, and therefore limiting the number to the

old


> > hard

> > > > limits (121 for 2K blocks, 504 for 8K blocks and so on) is still not

a


> > bad

> > > > idea).

> > > >

> > > > Regards

> > > > HJR

> > > > --

> > > > ----------------------------------------------

> > > > Resources for Oracle: http://www.hjrdba.com

> > > > ===============================

> > > >

> > > > "damorgan" <dan.morgan_at_ci.seattle.wa.us> wrote in message

> > > > news:3C829FD7.51727A5B_at_ci.seattle.wa.us...

> > > > > There is no such thing. Extent sizes are determined by developers

> and

> > DBAs

> > > > > that do their homework and determine the idea size for a class of

> > tables.

> > > > > Often ... creating multiple tablespaces with different extent

sizes


> to

> > > > avoid

> > > > > both fragmentation and keep the number of segments small.

> > > > >

> > > > > Daniel Morgan

> > > > >

> > > > >

> > > > >

> > > > > EP wrote:

> > > > >

> > > > > > Hi

> > > > > >

> > > > > > Does anybody know the algorithm used by Oracle when calculating

> the

> > > > extent

> > > > > > size for LMT when autoallocate option is specified ???

> > > > > >

> > > > > > TIA

> > > > > > EP

> > > > >

> > >

> >

> >

>

>