Re: Algorithm for calculating extent size in LMT

In article <a61g2r$ubr$1_at_lust.ihug.co.nz>, "Howard says...
>
>I'm not going to debate the issue with you, Daniel, because the last time I
>tried reasoning with you, you claimed you'd created a database without a
>system rollback segment, and went mysteriously silent on the subject when
>asked to prove it.

Howard --

this is the holy grail. This entire topic is shrouded in so much mysticism and hocus pocus -- its silly after a while.

If I had a penny for every time I've said "so what if you have x hundred extents, no big deal". Having everything in one extent is not desirable. It is not bad to have lots of extents. Yadda Yadda Yadda and so on.... Well, I could easily retire many times over.

People just don't get it. Even when you show them -- they still don't believe you.

It is a shame, most mistakes I see are made by people trying to reorg in order to get rid of 5 extents and put it into that 'perfect' one extent table. They lose a grant, index or something else. They should just leave it be.

Its also funny to hear the "hey you want contigous extents so the head doesn't have to move on the disk". They may have been true in 1983 or something but today with disk arrays, volume managers, raid everywhere, MULTI-USER SYSTEMS (this one just totally kills that head moving theory), and so on -- well, you get the picture.

>
>If you (or anyone else) would care to visit
>http://www.hjrdba.com/adminpdfs/contiguous.pdf, you'll find a testing script
>I knocked up to create a number of segments, comprising lots of small
>extents, non-contiguous; followed by lots of small extents, contiguous.
>Then it tried fewer large extents, non-contiguous, and finally fewer large
>extents, contiguous. You'll also find the timings for full tablescans and
>complete deletes in that document. By way of a summary (in case you don't
>want to read the thing in detail), selecting and deleting half a million
>rows under various extent configurations went like this:
>
>Many small extents : Select Discontiguous - 11.78 seconds
>Many small extents : Select Contiguous - 12.06 seconds
>Many small extents : Delete Discontiguous - 63.04 seconds
>Many small extents : Delete Contiguous - 63.81 seconds
>
>Few large extents : Select Discontiguous - 12.17 seconds
>Few large extents : Select Contiguous - 10.41 seconds
>Few large extents : Delete Discontiguous - 62.79 seconds
>Few large extents : Delete Contiguous - 66.28 seconds
>
>You tell me where the performance difference arises between any of these
>tests, particularly given the natural degree of variation between and within
>test runs anyway.
>
>There isn't one.
>
>HJR
>--
>----------------------------------------------
>Resources for Oracle: http://www.hjrdba.com
>===============================
>
>
>"damorgan" <dan.morgan_at_ci.seattle.wa.us> wrote in message
>news:3C83CB94.496FA07_at_ci.seattle.wa.us...
>> And I agree with everything you said.
>>
>> Thank you,
>>
>> Daniel Morgan
>>
>>
>>
>> dkrpata1 wrote:
>>
>> > 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
>> > > > > >
>> > > >
>> > >
>> > >
>>
>
>

--
Thomas Kyte (tkyte@us.oracle.com)             http://asktom.oracle.com/ 
Expert one on one Oracle, programming techniques and solutions for Oracle.
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