ASM Metadata Internals

A collection of experiments and facts on Oracle ASM metadata internals. More on configuration and performance at HAandPerf

ASM metadata from V$ and X$ views

ASM instance V$ views and X$ tables:

View Name	X$ Table name	Description
V$ASM_DISKGROUP	X$KFGRP	performs disk discovery and lists diskgroups
V$ASM_DISKGROUP_STAT	X$KFGRP_STAT	ists diskgroups
V$ASM_DISK	X$KFDSK, X$KFKID	performs disk discovery, lists disks and their usage metrics
V$ASM_DISK_STAT	X$KFDSK_STAT, X$KFKID	lists disks and their usage metrics
V$ASM_FILE	X$KFFIL	lists ASM files, including metadata/asmdisk files
V$ASM_ALIAS	X$KFALS	lists ASM aliases, files and directories
V$ASM_TEMPLATE	X$KFTMTA	lists the available templates and their properties
V$ASM_CLIENT	X$KFNCL	lists DB instances connected to ASM
V$ASM_OPERATION	X$KFGMG	lists rebalancing operations
N.A.	X$KFKLIB	available libraries, includes asmlib path
N.A.	X$KFDPARTNER	lists disk-to-partner relationships
N.A.	X$KFFXP	extent map table for all ASM files
N.A.	X$KFDAT	extent list for all ASM disks

This list is obtained querying v$fixed_view_definition where view_name like '%ASM%' which exposes all the v$ and gv$ views with their definition. Additionally querying v$fixed_table where name like 'X$KF%' and name like 'X$KF%' (ASM views start with the KF prefix).

Oracle*NET to ASM

TIP: An example of tnsnames entry to be used to connect to ASM instances via Oracle*NET (you will also need a password file of course). Note the extra keyword (UR=A). More generally UR=A allows to connect to 'blocked services'.

ASM1 =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = [hostname])(PORT = [portN]))
    (CONNECT_DATA =
      (SERVER = DEDICATED) (SERVICE_NAME = +ASM)  (INSTANCE_NAME = +ASM1)
      (UR=A)
    )  )

Striping and Mirroring with ASM, extents and allocation units

An basic example, using ASM and normal redundancy: the available storage, say 64 HDs over FC SAN, are used to create the main DB diskgroup: DATADG. DATADG is logically divided into 2 evenly sized groups of disks: 32 disks in failgroup N.1 and 32 in failgroup N.2. Oracle datafiles created in DATADG are 'striped' into smaller pieces, extents of 1MB in size. Extents are allocated to the storage in 2 (mirrored) allocation units (AU): one AU in failgroup N.1 the other in failgroup N.2.

Selected X$tables

X$KFFXP

This X$ table contains the mapping between files and used allocation units. It allows to track the position of all the extents of a given file striped and mirrored across storage. Read operations are load-balanced over primary and mirror allocation units (for performance reasons). Write operations need instead to write both AU to disk.

X$KFFXP Column Name	Description
ADDR	x$ table address/identifier
INDX	row unique identifier
INST_ID	instance number (RAC)
NUMBER_KFFXP	ASM file number. Join with v$asm_file and v$asm_alias
COMPOUND_KFFXP	File identifier. Join with compound_index in v$asm_file
INCARN_KFFXP	File incarnation id. Join with incarnation in v$asm_file
PXN_KFFXP	file AU number
XNUM_KFFXP	ASM file extent number (mirrored AU have the same extent value)
GROUP_KFFXP	ASM disk group number. Join with v$asm_disk and v$asm_diskgroup
DISK_KFFXP	Disk number where the extent is allocated. Join with v$asm_disk
AU_KFFXP	Relative position of the allocation unit from the beginning of the disk. The allocation unit size (1 MB) in v$asm_diskgroup
LXN_KFFXP	0=primary extent, 1=mirror extent, 2=2nd mirror copy (high redundancy)
FLAGS_KFFXP	N.K.
CHK_KFFXP	N.K.

X$KFDAT

This X$ table contains details of all allocation units (free and used).

X$KFDAT Column Name	Description
ADDR	x$ table address/identifier
INDX	row unique identifier
INST_ID	instance number (RAC)
GROUP_KFDAT	diskgroup number, join with v$asm_diskgroup
NUMBER_KFDAT	disk number, join with v$asm_disk
COMPOUND_KFDAT	disk compund_index, join with v$asm_disk
AUNUM_KFDAT	Allocation unit identifier, join with x$kffxp.au_kffxp
V_KFDAT	V=this Allocation Unit is used; F=AU is free
FNUM_KFDAT	file number, join with v$asm_file
I_KFDAT	N.K.
XNUM_KFDAT	ASM file AU number join with x$kffxp.pxn_kffxp
RAW_KFDAT	raw format encoding of the disk,and file extent information

X$KFDPARTNER

This X$ table contains the disk-to-partner (1-N) relationship. Two disks of a given ASM diskgroup are partners if they each contain a mirror copy of the same extent. Therefore partners must belong to different failgroups of the same diskgroup. From a few live examples I can see that typically disks have 10 partners each. This mechanism is in place to reduce the chance of losing both sides of the mirror in case of double disk failure.

X$KFDPARTNER Column Name	Description
ADDR	x$ table address/identifier
INDX	row unique identifier
INST_ID	instance number (RAC)
GRP	diskgroup number, join with v$asm_diskgroup
DISK	disk number, join with v$asm_disk
COMPOUND	disk identifier. Join with compound_index in v$asm_disk
*NUMBER_KFDPARTNE*R	partner disk number, i.e. disk-to-partner (1-N) relationship
MIRROR_KFDPARNER	=1 in a 'clean configuration'
PARITY_KFDPARNER	=1 in a 'clean configuration'
ACTIVE_KFDPARNER	=1 in a 'clean configuration'

X$KFFIL and metadata files

..to be edited

Examples

...to be edited, see also https://twiki.cern.ch/twiki/pub/PSSGroup/HAandPerf/ASM_metadata_30012006.html

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Revisions:
V1.0 Jan 2006, Luca.Canali@cernNOSPAMPLEASE.ch
Major additions, Jan2007, L.C.