Will a converted LMT have UNIFORM EXTENT MANAGEMENT?

if we convert a DMT to an LMT, can we then set this LMT to have UNIFORM EXTENT MANAGEMENT?
thanks

Don't convert. Yes, Oracle provides a procedure to do the conversion, but it ends up creating a horrible 'hybrid' sort of tablespace, certainly unable to use a uniform extent management technique, but not really using a proper autoallocate method either.
Better to create a new tablespace and move segments across from one to the other. Then drop the original.

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AUTOALLOCATE or UNIFORM extents for LMTs?

Hi,
I checked metalink and other resources, but couldn't find a conclusive statement indicating as to which one of the following is the best and recommended option:
1) LMTs with AUTOALLOCATE or
2) LMTs with UNIFORM EXTENT sizes
Any pros and cons would be appreciated.
Thanks
SS

A classic example where AUTOALLOCATE is very good is the 3rd party application where you don't know which tables are going to be big, which ones small, and which one won't be used at all If you put everything into AUTOALLOCATE it avoids wasting space, but lets the big tables grow without producing a vast number of extents.
If you are in control of the application and have good information about sizing of a few critical objects, you might choose to use UNIFORM sizing for adminstrative and monitoring reasons - it gives you the option for watching objects grow at a predictable rate.
If you are doing a lot of parallel work with scratch tables (CTAS, insert /*+ append */ then you might want to read up about the possible conflict between PX and AUTOALLOCATE at the following URL:
http://jonathanlewis.wordpress.com/2007/05/29/autoallocate-and-px/
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http://jonathanlewis.wordpress.com
http://www.jlcomp.demon.co.uk

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Why use uniform extent allocation?

version- 11.2.0.2.0
Hello guys, I've been reading UNIFORM vs AUTOALLOCATE extent allocation.
I've read the following articles.
https://blogs.oracle.com/datawarehousing/entry/parallel_load_uniform_or_autoallocate
Ask Tom: On Loading and Extents
https://forums.oracle.com/thread/2518951
From what I understood, autoallocate trumps the uniform in all scenarios (unless I am missing something).
In the thread "AUTOALLOCATE vs UNIFORM SIZE"
for the benefits of autoallocate and uniform size allocation Kh$n wrote
Benefits of AUTOALLOCATE
* Prevents space fragmentation.
Benefits of UNIFORM extent sizes
* Prevents fragmentation.
(I dont understand what is the difference between those two fragmentation prevention, are those benefits one and the same?)
even in scenarios where we know exactly how much data will be loaded, there is always a chance of extent wastage and with out extent trimming that space will be unusable.
Can someone please explain in which cases we use uniform extent allocation?
for suppose we use uniform extent allocation and we have lot of unused space from the extent allocation, can that space be reclaimed using shrink space command for tables and indexes?
Thank You

Extent trimming, to the best of my knowledge, is something that only happens when you are using parallel query to do large loads, not something that happens during normal OLTP type operations. As with anything called "automatic" in Oracle, though, the internals are subject to change across versions (and patchsets) and are not necessarily documented, so it is entirely possible for behaviors to change over time. Relying on specific internal behaviors is generally not a good idea.
The example I gave (assuming you reverse the truncating of A and the loading of C, as Hemant pointed out) produces "fragentation" when you're using automatic extent management. It's not a particularly realistic scenario, but it is possible. If you never delete data, never truncate tables, (and, presumably, never shrink tables), extents would never be deallocated and there would, therefore, never be holes. That is just as true of ancient dictionary managed tablespaces as well as locally managed tablespaes whether you're using uniform or autoallocated extents.
Shrinking a table has nothing to do with defragmenting a tablespace. It is simply compacting the data in the table and then potentially deallocating extents. You can do that with any locally managed tablespace. There is still the possibility, of course, that you have just enough data in the table that you need to allocate 1 extra extent when you only need space for 1 row in 1 block. So there may be some number of MB of "wasted" space per segment (though, again, this is generally not something that is a practical concern since the data in tables generally changes over time and it's generally not worth the effort of worrying about a few MB).
Justin
For your third question, assuming both extents are part of the same segment, assuming that the space is actually usable based on things like the PCTUSED setting of the table, and assuming a nice, simple conventional path insert in a single-user, Oracle would use the free space in the extent for new inserts before allocating a new extent. Oracle generally doesn't allocate new extents unless it needs to (there are caveats to this-- if the only blocks with free space have a relatively large fraction of their space used such that a particular new insert only fits in 1 of the 1 million blocks in the currently allocated extents, Oracle will potentially give up before finding the 1 in a million block that it would need an may allocate a new extent).
Message was edited by: JustinCave

Large Uniform Extent Size = Slow TRUNCATE?

Here's the scenario...
We have a a tablespace with the following storage parameter:
EXTENT MANAGEMENT LOCAL UNIFORM SIZE 32M
Users were complaining about slow TRUNCATE performance. I saw the same when I created a table with 30,000 rows - same as the user was complaining about - in the same tablespace.
I proceeded to move the objects from the schema the user was referencing to a tablespace with:
EXTENT MANAGEMENT LOCAL AUTOALLOCATE
... and the TRUNCATE executed in the expected time (less than a second) for the same amount of rows in the same table structure.
Why does a large UNIFORM extent size (such as 32M in this case) cause for slow TRUNCATE performance? I wasn't able to find an exact cause of this in the forums or on Metalink thus far.
Version: Oracle DB 10.2.0.3
System Info:
Linux ilqaos01c 2.6.9-55.0.12.ELsmp #1 SMP Wed Oct 17 08:15:59 EDT 2007 x86_64
Thanks.

Robert Sislow wrote:
The Metalink article was helpful, however, the database we're on is version 10.2.0.3, and the article is referencing 9.2.0.4.
Additionally, the last few responses in this thread are referring to concurrent TRUNCATE operations. The TRUNCATE that we're running is a single-thread TRUNCATE on a very small table - about 8000 rows.
After executing a 10046 level 12 trace, and using the Trace Analyzer tool, we've found that the "local write wait" event is taking up ~90% of the statement's activity for each run. Once again, all we can find that's causing this is the fact the the extent size in the tablespace with the table where we're seeing the slowness is set to UNIFORM size of 32M.
You're using ASSM (automatic segment space management), which means you have a number of bitmap space management blocks scattered through the object.
If you're running with 32MB uniform extents, the first extent will be 4096 blocks, and there will be one level 2 bitmap, 64 level 1 bitmaps, and the segment header block at the start of the extent. With autoallocate, the first extent will start with one level 2 bitmap, one (or possibly 2) level 1 bitmap(s) and the segment header block.
When you truncate an object, all the space managment blocks in the first extent (and any extents you keep) have to be reset to show 100% free space - this means they may all have to be read into the buffer cache before being updated and written back with local writes (i.e. writes by the process, not by dbwr).
So you have to wait for 66 reads and writes in one case and 3 (or 4) reads and writes in the other case. This helps to explain part of the difference. However, a local write wait should NOT take the best part of a second - so there must be a configuration problem somewhere in your setup. (e.g. issues with async I/O, or RAID configuration).
Regards
Jonathan Lewis

Different between datafile autoextent and tablespace extent management auto

hi guys,
the above got me thinking.
blocks are arrange in extents. The rest is ???? //??? ... . .... ..

flaskvacuum wrote:
hi guys,
the above got me thinking.
blocks are arrange in extents. The rest is ???? //??? ... . .... .. Datafile autoextend (its not datafile auto extent) - It will increase datafile size when all space in datafile is occupied and all its extents are used. So when objects needs to grow, oracle will increase the size of datafile (if autoextend is ON) and allocate extents to objects.
Also see http://docs.oracle.com/cd/B28359_01/server.111/b28310/dfiles003.htm
Tablespace extent management auto - Local managed tablespace will keep the track of extents in Bitmaps. Now creating this type of tablespace you can specify AUTOALLOCATE clause or the UNIFORM clause, AUTOALLOCATE will assign new extents to obejcts of size determine by oracle internally. UNIFORM if specified, oracle will assign uniform size extents to objects. Lets say you have specified 1M of Uniform size, then oracle will assign 1M of new extents to objects, all assigned extents will have same size i.e 1M. But its not the case with Autoallocate.
Autoallocate will determine whats the best extent size which he needs to allocate to segement. Its internal mechanism
http://docs.oracle.com/cd/B19306_01/server.102/b14231/tspaces.htm

Convert from dictionary to local managed?

Hi all,
Good day!
when we convert from dictionary to local managed is it dependant on CPU or no of extent?
Thanks in advance
Elina

How are you converting the tablespaces? 8 hours seems excessive if you are using dbms_space_admin.TABLESPACE_MIGRATE_TO_LOCAL , I have seen situations where creating new locally managed tablespaces and moving the objects accross takes a long time as the old extnets are deallocated from the UET$ table and added to FET$. in these cases the speed of an individual CPU core is the limiting factor as SMON handles the deallocation of extents.
If this is the case then migratig the tablespace to locally managed using dbms_space_admin then migrating to a new tablespace may be faster, if not then can you post what method you are uing to migrate and the version of the database
Chris

EXTENT MANAGEMENT

Hi,
on 11g R2 on Win 2008
Would it be better to change to UNIFORM SIZE 128K for performance ?
What are the advantage of AUTOALLOCATE ?
Can we alter from AUTOALOCATE to UNIFORME or vice versa?
Thank you.

user522961 wrote:
Hi,
on 11g R2 on Win 2008
Would it be better to change to UNIFORM SIZE 128K for performance ?
What are the advantage of AUTOALLOCATE ?
Can we alter from AUTOALOCATE to UNIFORME or vice versa?
Thank you. What made you think so to change to UNIFORM size?
There is lots of debates around the AUTOALOCATE vs UNIFORME extents size performance, because its not like one size fits all. So you have to carefully read your current setup/configuration of database first then plan to change to UNIFORM.
UNIFORM extents are choosen for Large segments when you know its size in advance and you are inserting large amount of data.
However if you are not sure about size of segments and you are unaware of data that will be manipulated then i would suggest you to go with AUTO, this is usually used in case where future growth is not known and data set or segments are small in size typically in OLTP systems.
See nice blog here https://blogs.oracle.com/datawarehousing/entry/parallel_load_uniform_or_autoallocate
And - http://www.oracle.com/technetwork/issue-archive/2007/07-may/o37asktom-101781.html

Difference between extent management and segment space management

Hello,
Could you tell me the difference between extent management and segment space management interms of datablocks?

After 10g, default tablespace is locally managed and with this creation, your segment's spaces are managed "automatically".
Automatic segment space management is onlyt avaliable to locally managed tablespaces.
The logic of the automatic segment space management is;
For instance, we have a rectangle cube which is totally empty (1 extent - 8 segments - 64 block in default). If we enter a new data to our table, data is stored inside this cube with it's information. (default 8k). If our data must be splitted, check row chaining for further information.
At this point, oracle decides which block is filled with the entered data. It automatically fills the emty blocks. You can use this only if your tablespace is locally managed and your segment space managemet is auto.
On the other hand,
If we create dictionary managed tablespace,
Data is stored in "dictionary" (tables in SYS schema). This is a bit slower than locally managed tablespaces. So filling the extents, segments and blocks will be slow too. You need to specify the PCTUSED, FREELISTS, FREELISTS GROUPS for the storage of the data. If segment space managent is auto, you don't need to define those values.
In summary, locally managed tablespaces with automatic segment space management is Oracle's maybe the most important feature. I am not using dictionary managed tablespaces and i advice don't use it either. In the end, this is an upgrade of using segment spaces much more faster, easier and reliable, trustable.
PS: You will need to check "High Water Mark" for automatic segment space management.

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Will a converted LMT have UNIFORM EXTENT MANAGEMENT?

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