2.2.2.1 About Fast Disk Scan Rates

To achieve fast disk scan rates, it is important to lay out segments with at least 4 MB of contiguous space. This allows disk scans to read 4 MB of data before performing another seek at a different location on disk. To ensure segments are laid out with 4 MB of contiguous space, set the Oracle ASM allocation unit size to 4 MB, and ensure data file extents are also at least 4 MB. The allocation unit can be set with the disk group attribute AU_SIZE when creating the disk group.

The following SQL command creates a disk group with the allocation unit set to 4 MB. The compatible.rdbms attribute is set to 11.2.0.2 in order to support both release 11.2.0.2 and release 11.2.0.3 databases in a consolidated environment.

SQL> CREATE DISKGROUP data NORMAL REDUNDANCY 
     DISK 'o/*/data_CD*'
     ATTRIBUTE 'compatible.rdbms' = '11.2.0.2', 
               'compatible.asm' = '11.2.0.3',
               'content.type' = 'data',
               'cell.smart_scan_capable' = 'TRUE',
               'au_size' = '4M';