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fragmentation

Reorg

A current question on the OTN database forum asks: “What’s the difference between object and tablespace reorganization?” Here’s an analogy to address the question.

I have three crates of Guiness in the boot (trunk) of my car, one crate has 4 bottles left, one has 7 bottles left and one has 2 bottles. I also have two cases of Louis Roederer Brut NV champagne, one case has 2 bottles left and one has only one. (I have two objects in my tablespace – one of type Beer, one of type Champagne – and my boot requires manual free space management .)

I move all the Guiness bottles into a single crate and all the champagne bottles into a single case. That’s a couple of “shrink space compact” calls – I’ve re-organised the objects to get all the bottles in each object close to each other, but the crates are still taking up space in the boot.

Securefile space

Here’s a little script I hacked together a couple of years ago from a clone of a script I’d been using for checking space usage in the older types of segments. Oracle Corp. eventually put together a routine to peer inside securefile LOBs:

Space Usage

Here’s a simple script that I’ve used for many years to check space usage inside segments.  The comment about freelist groups may be out of date  – I’ve not had to worry about that for a very long time. There is a separate script for securefile lobs.

Shrink Tablespace

If you start moving objects around to try and reclaim space in a tablespace there are all sorts of little traps that make it harder than you might hope to get the maximum benefit with the minimum effort.  I’ve written a couple of notes in the past about how to proceed and, more recently, one of the difficulties involved. This is just a brief note about a couple of ideas to make life a little easier.

Shrink Tablespace

In a comment on my previous post on shrinking tablespaces Jason Bucata and Karsten Spang both reported problems with small objects that didn’t move to the start of the tablespace. This behaviour is inevitable with dictionary managed tablespaces (regardless of the size of the object), but I don’t think it’s likely to happen with locally managed tablespaces if they’ve been defined with uniform extent sizes. Jason’s comment made me realise, though, that I’d overlooked a feature of system allocated tablespaces that made it much harder to move objects towards the start of file. I’ve created a little demo to illustrate the point.

Shrink Tablespace

A recent question on the OTN database forum raised the topic of returning free space in a tablespace to the operating system by rebuilding objects to fill the gaps near the start of files and leave the empty space at the ends of files so that the files could be resized downwards.

This isn’t a process that you’re likely to need frequently, but I have written a couple of notes about it, including a sample query to produce a map of the free and used space in a tablespace. While reading the thread, though, it crossed my mind that recent versions of Oracle introduced a feature that can reduce the amount of work needed to get the job done, so I thought I’d demonstrate the point here.

Shrink Space

Here’s a lovely effect looking at v$lock (on 11.2.0.4)

Wasted Space

Here’s a little quiz: If I take the average row length of the rows in a table, multiply by the number of rows, and convert the result to the equivalent number of blocks, how can the total volume of data in the table be greater than the total number of blocks below the table high water mark ? I’ve got three tables in a schema, and they’re all in the same (8KB block, 1M uniform extent, locally managed) tablespace, but here’s a query, with results, showing their space utilisation – notice that I gather schema stats immediately before running my query:

ASM AU Size And LMT AUTOALLOCATE

When using Locally Managed Tablespaces (LMT) with variable, system managed extent sizes (AUTOALLOCATE) and data files residing in ASM the Allocation Unit (AU) size can make a significant difference to the algorithm that searches for free extents.The corresponding free extent search algorithm when searching for free extents >= the AU size seems to only search for free extents on AU boundaries in order to avoid I/O splitting.Furthermore the algorithm seems to use two extent sizes when searching for free extents: A "desired" (for example 8MB) and a "minimum acceptable" (for example 1MB) extent size - however when performing the search the "desired" size seems to be relevant when limiting the search to free extents on AU boundaries.This can lead to some surprising side effects, in particular when using 4MB AUs.It effectively means that although you might have plenty o

Free Space

Question – How can you have a single file in a single tablespace showing multiple free extents when there are no objects using any space in that file ? For example, from an 11.1.0.7 database:

SQL> select
  2          *
  3  from    user_free_space
  4  where
  5          tablespace_name = 'TEST_8K'
  6  order by
  7          file_id, block_id
  8  ;

TABLESPACE_N    FILE_ID   BLOCK_ID      BYTES     BLOCKS RELATIVE_FNO
------------ ---------- ---------- ---------- ---------- ------------
TEST_8K               3        128    1048576        128            3
TEST_8K               3        256    1048576        128            3
TEST_8K               3        384    1048576        128            3
TEST_8K               3        512  130023424      15872            3

4 rows selected.

The answer in this case is simple – here’s what I did just before running my query: