RE: Explanation of tbcheck -pt output

Hi, 1. Generally when an extent is added to a tablespace all pages are allocated but none of them is used - well except one (so called "bitmap page") which is used for tracking data pages. The database allocates the pages but yet doesn't use them. Does that make sense? The idea behind this is that an extent is always a continous space of the HDD thus access is much faster... 2. No idea. To my understanding "Pages used" should also increase... 3. Question of what you are interested about. If you wanna know the number of allocated pages the answer is "Number of pages allocated". If you wanna know how much the table actually "uses" (eg. stores data there) the answer is "Number of pages used". 4. Extent is automatically added to the tablespace when there are no more free pages, eg. all allocated pages are used, or in other words "Number of pages allocated= Number of pages used". The space is computed from the "Next extent size" parameter and the number of extents. The default value of the "Next extent size" is 8, but it could be also set in the CREATE TABLE statement. For instance: CREATE TABLE mytable(...) EXTENT SIZE 1000 NEXT SIZE 500; Note: in the CREATE TABLE statement, rather then pages, KByte is used to measure extent size. Thus, (on a 2K machine) mytable's next extent size will be 250 pages. When mytable is created 500 pages are allocated and none is used except one. When the tablespace fills up the database allocates more 250 pages. Then again when the 750 pages fill up the database allocates more 250 pages. etc. Note: with every 16th extent, the size is doubled. Thus for mytable: 1st extent size is 500 pages, 2nd extent size is 250 pages..., 17th extent size is 500 pages..., 33rd extent size is 1000 pages..., 49th extent size is 2000 pages..., etc. Good luck with your utility:) Csomi -----Original Message----- From: Jinu.Joseph@citicorp.com [mailto:Jinu.Joseph@citicorp.com] Sent: Saturday, December 16, 2000 9:47 PM To: engines@iiug.org; informix-client-tools@iiug.org; informix-list@iiug.org; linux-informix@iiug.org Subject: Explanation of tbcheck -pt output Hi, The following is the output of the command tbcheck -pt for a table in my database : ... ... Maximum row size 1549 Number of special columns 5 Number of keys 6 Number of extents 32 Current serial value 1 First extent size 460800 Next extent size 184320 Number of pages allocated 5312017 Number of pages used 5246787 Number of data pages 3134215 Number of data bytes 554916459 Number of rows 3134215 ... ... Need the answer for the following questions: 1. Why is there a difference between pages allocated and pages used ? 2. When monitoring on a daily basis only the value of data pages increases. Pages used stays constant. Why? 3. How do I find out the actual space usage of the table ? 4. When is the next extent added to a tblspace and how does it compute the space required ? I am trying to develop a utily that will send out a warning when the tablespace reaches a threshold limit of say 80% Thanks in advance Jinu Joseph