ASM Explorations

A collection of experiments and facts on ASM internals.
At CERN Physics DB we use Oracle 10g ASM (Automatic Storage Manager) as a cluster filesystem and volume manager for Oracle RAC. ASM is used to build 'SAME' (stripe and mirror everything') volumes from a number of low-cost storage arrays. Software mirroring by ASM is a key feature of the architecture Architecture_description.pdf ASM is managed via a specialized ASM instance, operations are done using special SQL commands, while metadata is visible via V$ and X$ views. 10.2.0.3 and normal redundancy is the reference for the examples below.

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: connecting to ASM instances via Oracle*NET requires an extra keyword (UR=A) in the tnsnames alias, or you will get an error message that the instance is blocked. You also need a password file Example:

ASM1 =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = [hostname])(PORT = [portN]))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = +ASM1)
      (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, is used to create a diskgroup DATADG. DATADG is built using 2 evenly sized groups of disks: 32 disks in failgroup N.1 and 32 in failgroup N.2. When a file is create in DATADG, it is logically divided in extents (1MB for datafiles) and allocated in 2 copies, called allocation units (AU): one AU in failgroup N.1 the other in failgroup N.2.

Selected X$tables

X$KFFXP

This table contains the mapping between files and allocation units. It allows to track the position of all the extents of a given file striped and mirrored across storage.

Columns 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	Extent number (for the given file)
XNUM_KFFXP	Logical extent number per file (mirrored extents have the same 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 file header/metadata

X$KFDAT

X$KFDPARTNER

X$KFFIL and metadata files