I have 40 Kilobytes as an extent size in my tablespaces. Now I read the following text on this page.
Having 1,000 extents for an object by itself does not pose any performance problems, so long as the extents are sized as a multiple of (DB_FILE_MULTIBLOCK_READ_COUNT * DB_BLOCK_SIZE). The aforementioned formula is relevant for full-table scans or index fast full scans and not for single-block index scans. If the extent sizes adhere to the above formula, it ensures that Oracle will issue the same number of read system calls regardless of whether the object has 1 extent or 1,000 extents. If the extents are not aligned with the aforementioned size, additional read system calls can cause unnecessary overhead on the I/O sub-system. In the bigger scheme of things, assuming worst-case scenario, go ahead and add one additional read system call per extent per heavily hit table/index in your database. If you have many hundreds of objects with 1000s of misaligned extents, it will create overhead on the I/O sub-system.
In my 8i database:
db_file_multiblock_read_count = 128
db_block_size = 8 Kilobytes
Does this mean that the minimal extent size must be 128 * 8 = 1024KB?
Help is very appreciated.
Erik
Last edited by efrijters; 09-25-2003 at 07:38 AM.
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Is my extent size (40KB) too small? Is it better for I/O performance to make it the size (or multiple) of DB_FILE_MULTIBLOCK_READ_COUNT * DB_BLOCK_SIZE ??
Erik
An expert is one who knows more and more about less and less until he knows absolutely everything about nothing.
the reason for the "extent size should be multiple of (block size*db_file_multiblock_read_count)" statement is that when oracle and the o/s combine to read large chunks of data, the number of bytes that will be read in a single operation will be the minimum of ...
So, in your case you have block size*db_file_multiblock_read_count = 1Mb (which maybe misleadingly high -- your o/s might not support a 1Mb iu/o operation). If you have a table of 2Mb you could potentially scan it in 2 i/o operations (provided the o/s will do so, of course). However if that table is divided into extents of 40kb then each i/o operation will be limited to 40kb, and it will take "25.6" (ie. 26) i/o operations to scan the table.
If you have tables that fit within a single 40kb extent then that's ok -- you'll get your single i/o to scan the entire table, but a full scan of a multi-extent table with extent size that is not an integer multiple of 1Mb (or whatever the maximum allowed by the o/s is) will suffer.
So, if I get this right: If you give "db_file_multiblock_read_count" an ubsurdly high value, it gets the maximum (an therefore optimal) value for the OS?
When I test this in my db:
SQL>alter session set db_file_multiblock_read_count = 500;
Session altered.
SQL>select value from v$parameter where name = 'db_file_multiblock_read_count';
VALUE
----------------------------
128
1 row selected.
SQL>alter session set events '10046 trace name context forever, level 8';
Session altered.
SQL> select /*+ FULL(a) */ count(*) from sys.source$ a;
1 select bytes/1024, count(*)
2 from dba_extents where owner='SYS' and
3 segment_name='SOURCE$'
4 group by bytes/1024
5* order by 1
SQL> /
BYTES/1024 COUNT(*)
---------- ----------
16 1
256 218
My conclusion/question:
So Oracle reads in chunks of 32 blocks * 8K (block size) = 256K.
But the tablespace SYSTEM has an extent size of 64KB.
Is this good? Isn't it better to have a 256K extent size in this case?
I want to completely understand this. So that's why I inserted this long example...
Hope you can help me, SlimDave (or anybody)
Thanks in advance!!
Erik
An expert is one who knows more and more about less and less until he knows absolutely everything about nothing.
Originally posted by efrijters But the tablespace SYSTEM has an extent size of 64KB.
Is this good? Isn't it better to have a 256K extent size in this case?
Erik
Its the Segment's Extent Size that matter not TS, as its just that TS's extent size will be used if you dont specify storage param while segment is created in it..
BTW, dont play with SYSTEM TS.
funky...
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I know it's all about the segment size. I create tablespaces with a specific extent size and all objects in that tablespace inherit that extent size. So the extent sizes are equal. That's clear to me.
I would really like to have an answer to the above questions.
The example shown was executed because of an example at the "AskTom"-site.
An expert is one who knows more and more about less and less until he knows absolutely everything about nothing.
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