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Where does the optimizer find details on disk RAID, stripe size and blocksize?

Oracle Database Tips by Donald BurlesonDecember 18, 2015

Question: I see from your details on the optimizer cost model that the SQL optimizer is aware of the details about a disk blocksize as well as the RAID level and striping details.  Where is this information stored in the data dictionary?

Answer:  As a review, the CBO gathers information from many sources, and he has the lofty goal of using DBA-provided metadata to always make the "best" execution plan decision, and this does include details on the external storage, including average I/O tunings and details on the RAID and stripe size::
 

Oracle uses data from many sources to make an execution plan

 

ASM X$ tables

Inside the ASM instance we see these V$ views and their underlying X$ fixed tables:

X$ Table v$ View

X$KFGRP

V$ASM_DISKGROUP

X$KFGRP_STAT

V$ASM_DISKGROUP_STAT

X$KFDSK 

V$ASM_DISK

X$KFKID V$ASM_DISK

X$KFDSK_STAT

V$ASM_DISK_STAT

X$KFKID V$ASM_DISK_STAT

X$KFFIL

V$ASM_FILE

X$KFALS

V$ASM_ALIAS

X$KFTMTA

V$ASM_TEMPLATE

X$KFNCL

V$ASM_CLIENT

X$KFGMG

V$ASM_OPERATION

X$KFENV V$ASM_ATTRIBUTE
X$KFNSDSKIOST V$ASM_DISK_IOSTAT

Source: CERN

From the docs, here are the main ASM V$ views and their underlying contents:

VIEW ASM INSTANCE DB INSTANCE
v$asm_diskgroup Describes a disk group (number, name, size related info, state, and redundancy type) Contains one row for every open ASM disk in the DB instance.
v$asm_client Identifies databases using disk groups managed by the ASM instance. Empty
v$asm_disk Contains one row for every disk discovered by the ASM instance, including disks that are not part of any disk group. Contains rows only for disks in the disk groups in use by that DB instance.
v$asm_file Contains one row for every ASM file in every disk group mounted by the ASM instance. Contains rows only for files that are currently open in the DB instance.
v$asm_template Contains one row for every template present in every disk group mounted by the ASM instance. Empty
v$asm_alias Contains one row for every alias present in every disk group mounted by the ASM instance. Empty
v$asm_operation Contains one row for every active ASM long running operation executing in the ASM instance. Empty

v$asm_attribute
All ASM configuration attributes  

v$asm_disk_iostat
Disk I/O statistics  

 

ASM internals and stripe size mapping

The stripe size for ASM data files is critical when estimating the cost of a full-table scan.

In these days when most Oracle databases adopt either ASM or RAID-10 (stripe and mirror everywhere) approach, every table is physically mapped across many physical disk drives and the internal mapping is hidden inside many layers:

Oracle Certified Master Steve Karam shares the internals between ASM and disk mapping:

See:  Mapping LUN and MAS to ASM

 

If you like Oracle tuning, you may enjoy my new book "Oracle Tuning: The Definitive Reference", over 900 pages of BC's favorite tuning tips & scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.



 

 

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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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