Specifying the Sector Size for Drives

You can use the optional SECTOR_SIZE disk group attribute with the CREATE DISKGROUP SQL statement to specify disks with the sector size set to the value of SECTOR_SIZE for the disk group. Oracle ASM provides support for 4 KB sector disk drives without negatively affecting performance. The SECTOR_SIZE disk group attribute can be set only during disk group creation.

The values for SECTOR_SIZE can be set to 512, 4096, or 4K if the disks support those values. The default value is platform dependent. The COMPATIBLE.ASM and COMPATIBLE.RDBMS disk group attributes must be set to 11.2 or higher to set the sector size to a value other than the default value.


Oracle Automatic Storage Management Cluster File System (Oracle ACFS) does not support 4 KB sector drives. There is a performance penalty for Oracle ACFS when using 4 KB sector disk drives in 512 sector emulation mode.

The following validations apply to the sector size disk group attribute.

  • Oracle ASM prevents disks of different sector sizes from being added to the same disk group. This validation occurs during CREATE DISKGROUP, ALTER DISKGROUP ADD DISK, and ALTER DISKGROUP MOUNT operations.

  • If the SECTOR_SIZE attribute is explicitly specified when creating a disk group, then Oracle ASM attempts to verify that all disks discovered through disk search strings have a sector size equal to the specified value. If one or more disks were found to have a sector size different from the specified value, or if Oracle ASM was not able to verify a disk sector size, then the create operation fails.

    Oracle ASM also attempts to verify disk sector size during the mount operation and the operation fails if one or more disks have a sector size different than the value of the SECTOR_SIZE attribute.

  • If the SECTOR_SIZE attribute is not specified when creating a disk group and Oracle ASM can verify that all discovered disks have the same sector value, then that value is assumed for the disk group sector size that is created. If the disks have different sector sizes, the create operation fails.

  • When new disks are added to an existing disk group using the ALTER DISKGROUP .. ADD DISK SQL statement, you must ensure that the new disks to be added have the same value as the SECTOR_SIZE disk group attribute. If the new disks have different sector sizes, the alter operation fails.

  • You can determine the sector size value that has either been assumed or explicitly set for a successful disk group creation by querying the V$ASM_ATTRIBUTE view or run the ASMCMD lsattr command. You can also query the SECTOR_SIZE column in the V$ASM_DISKGROUP view.

         WHERE name = 'sector_size' AND group_number = 1;
    NAME                        VALUE
    --------------------------- -----------------------
    sector_size                 512
    SQL> SELECT group_number, sector_size FROM V$ASM_DISKGROUP 
         WHERE group_number = 1;
    ------------ -----------
               1         512
  • Not all disks support all of the possible SECTOR_SIZE values. The sector size attribute setting must be compatible with the physical hardware.

As shown in Example 4-3, you can use the SECTOR_SIZE attribute with the CREATE DISKGROUP SQL statement to specify the sector size of the disk drive on which the Oracle ASM disk group is located.

See Also:

Example 4-3 Creating a disk group of 4K sector size

FAILGROUP controller1 DISK
FAILGROUP controller2 DISK
ATTRIBUTE 'compatible.asm' = '11.2', 'compatible.rdbms' = '11.2',