ASM new features in Oracle Database 11g

ASM was first time introduced in Oracle Database 10g. Since then lot of improvement has been done. Here I am presenting changes and new feature when you use ASM in Oracle Database 11h environment.

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The default parameter settings work perfectly for ASM. The only parameters needed for 11g ASM:
ASM_DISKSTRING
ASM_DISKGROUPS
INSTANCE_TYPE

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ASM path are not part of regular OS level I/O. Because of this RDBMS does a raw I/O which will be faster as compared to file system I/O. Since there is no intervention OS.

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ASM Fast Mirror Resync - When first time ASM features was released in 10g, Whenever it is unable to complete the write or read operation in a disk, disk was dropped from ASM group. This was relatively costlier operation. It could take hours to do that.
but now in 11g, When a disk goes offline following a transient failure, ASM tracks the ASM data extents that are modified during the outage. After the transient failure is repaired, ASM quickly resynchronizes only those ASM data extents that have been affected during the outage. This feature assumes that the content of the affected ASM disks has not been damaged or modified.
conclusion is this feature is beneficial by reducing the time required to resynchronize a transient failure of a disk.

ALTER DISKGROUP dgroupA SET ATTRIBUTE 'DISK_REPAIR_TIME'='4H';
ALTER DISKGROUP dgroupA
OFFLINE DISKS IN FAILGROUP site2 DROP AFTER 6H;
ALTER DISKGROUP dgroupA
ONLINE DISKS IN FAILGROUP site2 POWER 2 WAIT;
ALTER DISKGROUP dgroupA DROP DISKS IN FAILGROUP site2
FORCE;

V$ASM_ATTRIBUTE,V$ASM_DISK, V$ASM_DISK_IOSTAT,V$ASM_OPERATION views can be checked for further investigation.

ASM Preferred Mirror Read
============================================
Mirroring is done to ensure the protection from data loss. ASM failure groups in Oracle Database 10g always reads the primary copy of a mirrored ASM data extent. If a node is allowed to read the data from local or from closest node inspite that data is secondary, then it can improve the performance by reducing network traffic. Oracle Database 11g can do this by configuring preferred mirror read using the new ASM_PREFERRED_READ_FAILURE_GROUPS initialization parameter to specify a list of preferred mirror read names.

ASM_PREFERRED_READ_FAILURE_GROUPS=DATA.SITEA On first instance
ASM_PREFERRED_READ_FAILURE_GROUPS=DATA.SITEB On second instance

This can be monitored from v$asm_disk(preferred_read) and v$asm_disk_iostat tables.

ASM Scalability and Performance Enhancements
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ASM uses a predetermined number of ASM data extents of each size. As soon as a file crosses a certain threshold, the next ASM data extent size is used. An ASM file can begin with 1 AU; as the file’s size increases, the ASM data extent size also increases to 8 or 64 AUs based on predefined file size thresholds.

ASM also automatically defragments during allocation if the desired size is unavailable, thereby potentially affecting allocation times but offering much faster file opens, given the reduction in the memory required to store file ASM data extents.

SYSASM privilege
=======================================================
In Oracle Database 11g, if you log in to an ASM instance as SYSDBA, warnings are written in the corresponding alert.log file.
SYSASM privilege is replacing SYSDBA privilege as compared to proir version. SYSDBA privilege in ASM instance will be restricted.

ASM Disk Group Compatibility
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An ASM instance can support different RDBMS clients running at different compatibility settings. The database compatible version setting of each instance must be greater than or equal to the RDBMS compatibility of all disk groups used by that database. Database instances are typically run from a different Oracle home than the ASM instance. This implies that the database instance may be running a different software version than the ASM instance. When a database instance first connects to an ASM instance, it negotiates the highest version that they both can support.
Main Objective behind this new feature is to ensure heterogenous environment should also supproted by ASM.

Few New Maintenance command in ASM
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ALTER DISKGROUP DATA CHECK; Checks all the metadata directories by default
ALTER DISKGROUP data MOUNT RESTRICT; When a disk group is mounted in RESTRICTED mode, clients cannot access the files in a disk group.
ALTER DISKGROUP data MOUNT FORCE; Mounts the disk group even if some disks belonging to the disk group are not accessible
DROP DISKGROUP data FORCE INCLUDING CONTENTS; Enables users to drop a disk group that cannot be mounted; fails if the disk group is mounted anywhere

ASMCMD is now more powerful in 11g ASM
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The lsdsk command lists ASM disk information. This command can run in two modes:
• Connected mode: ASMCMD uses the V$ and GV$ views to retrieve disk information.
• Nonconnected mode: ASMCMD scans disk headers to retrieve disk information, using an ASM disk string to restrict the discovery set. The connected mode is always attempted first.

MD_BACKUP command is used to do backup of metadata from ASM disk group.
MD_RESTORE command is used to restore the lost metadata to ASM disk group.

ASMCMD> md_backup –b jfv_backup_file -g data
Disk group to be backed up: DATA#
Current alias directory path: jfv
ASMCMD>
ASMCMD> md_restore -b jfv_backup_file -t full -g data
Disk group to be restored: DATA#
ASMCMDAMBR-09358, Option -t newdg specified without any override options.
Current Diskgroup being restored: DATA
Diskgroup DATA created!
User Alias directory +DATA/jfv
created!

ASMCMD now support cp command by which we can copy the ASM files from local to remote node or vice versa and from file system to ASM disk or vice versa.
cp +DATA/ORCL/DATAFILE/TBSJFV.256.629730771 +DATA/ORCL/tbsjfv.bak
cp /home/oracle/tbsjfv.dbf +data/jfv
cp +DATA/orcl/datafile/tbsjfv.256.629989893 \sys@edcdr12p1.+ASM2:+D2/jfv/tbsjfv.dbf

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