Oracle Scratchpad

December 17, 2013

dbms_space usage

Filed under: ASSM,Indexing,Infrastructure,Oracle — Jonathan Lewis @ 6:43 pm BST Dec 17,2013

Just throwing out a brief comment (one of my many draft notes that I don’t have time to complete) about the dbms_space package. You’re probably familiar with this package and how, for ASSM segments, it can give you a measure of the available space in the blocks in a data segment, reporting 6 possible states of the blocks below the high high water mark (HHWM) e.g.:

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July 30, 2013

ASSM

Filed under: ASSM,Infrastructure,Oracle — Jonathan Lewis @ 6:53 am BST Jul 30,2013

A couple of thoughts.

The intent of ASSM is to minimise contention when multiple small transactions are busy inserting data concurrently into the same table. As a consequence, you may be able to find a number of odd behaviour patterns if you do experiments with a single session running one transaction at a time; or when executing a single large transaction, or when experimenting with small tables.

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May 20, 2011

ASSM

Filed under: ASSM,Infrastructure,Oracle — Jonathan Lewis @ 5:16 pm BST May 20,2011

Here’s an interesting little detail (obvious AFTER the event) about space management with ASSM (automatic segment space management). It starts with this question on OTN:

When I alter table deallocate unused and keep 1K the object ends up with 24 blocks, even after I’ve truncated the table. Why?
This is in a tablespace using ASSM, with locally managed extents set to use automatic (system) allocation.

Ultimately the answer is – the first extent in this table started life at 8MB, and an extent that large needs to have 16 level 1 bitmap (space management) blocks, one level 2 bitmap block, and the segment header block before you get to data blocks. When you truncate and deallocate Oracle doesn’t recreate the map, so the extent has to start with 18 blocks – round that up to the multiple of 8 blocks (the 64KB that Oracle normally uses for starting extents for small objects) and you get the 24 blocks from the question.

It took us a bit of time to get to the right answer on the thread – and that’s why I’m giving you the quick answer.

March 30, 2011

ASSM wreck

Filed under: ASSM,Bugs,Index Rebuilds,Indexing,Oracle,Troubleshooting — Jonathan Lewis @ 5:25 pm BST Mar 30,2011

Yesterday I introduced a little framework I use to avoid the traps inherent in writing PL/SQL loops when modelling a session that does lots of simple calls to the database. I decided to publish the framework because I had recently come across an example where a series of SQL statements gives a very different result from a single PL/SQL block.

 

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March 18, 2011

ASSM ouch!

Filed under: ASSM,Infrastructure,Oracle — Jonathan Lewis @ 6:20 pm BST Mar 18,2011

Here’s a nasty little surprise I got last week while investigating an oddity with stats collection. I wanted to create a table in an ASSM tablespace and populate it from two or three separate sessions simultaneously so that I could get some “sparseness” in the data load. So I created a table and ran up 17 concurrent sessions to insert a few rows each. Because I wanted to know where the rows were going I got every session to dump the bitmap space management block at the start of the segment – the results were surprising.
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January 19, 2011

ASSM Again

Filed under: ASSM,Infrastructure,Oracle,Troubleshooting — Jonathan Lewis @ 6:21 pm BST Jan 19,2011

While checking my backlog of drafts (currently 75 articles in note form) I came across this one from August 2009 and was a little upset that I hadn’t finished it sooner – it’s a nice example of geek stuff that has the benefit of being useful.

From the Oracle newsgroup comp.databases.oracle.server, here’s an example of how to recreate a performance problem due to maintenance on ASSM bitmaps in 10.2.0.4.

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July 19, 2010

Fragmentation 3

Filed under: ASSM,fragmentation,Infrastructure,Oracle — Jonathan Lewis @ 7:05 pm BST Jul 19,2010

This note is part three of a four-part series, and covers Table fragmentation. The whole series is as follows

  1. Introduction – with links to parts 2 – 4
  2. Disk and Tablespace Fragmentation
  3. Table Fragmentation – this bit
  4. Index Fragmentation

3. Table “fragmentation”.

In the introduction we discussed one type of table fragmentation that doesn’t (usually) matter – the fragmentation of a table into multiple extents. Here’s a funny thought – ASSM (automatic segment space management) introduces another form of table fragmentation that usually doesn’t matter.

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September 25, 2009

Bugs

Filed under: ASSM,Block Size,Bugs,Infrastructure,Oracle,Troubleshooting — Jonathan Lewis @ 6:03 pm BST Sep 25,2009

There are times as I browse through Metalink when I see descriptions of bugs that make me wonder how on earth anyone managed to find them. There are bugs which are so bizarre in their combination of prerequisites that you might think they’d never,ever, ¬†show up. Here’s one that got a mention on OTN some time back.

Problem: an update on a simple, unindexed, table takes 90 minutes if the table is in a tablespace using a 16KB block size; but closer to 90 seconds if the table is in a tablespace using a 4KB block size. The effect is totally reproducible.
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