This post is about ASM’s attribute deep knowledge. Before going ahead we must have an idea about the topic. Let’s some line must explain about  ASM -(Sector Disk Drives).

The database and Oracle’s Automatic Storage Management (ASM) volume manager explicitly understood 4k devices. In this and subsequent versions of ASM, disk groups created on native mode 4k devices have a “SECTOR_SIZE” property of 4096 (rather than the default 512), while for the database it became possible to change the default block size of the online redo log. However, at least in Linux, native mode is only possible through the use of the ASMLib kernel driver (discussed below). If you want to use ASM and native mode together, ASMLib is the only way. If you are one of the brave souls still using a filesystem rather than ASM then the native mode is possible, but only without DIRECTION enabled (through the use of the FILESYSTEMIO_OPTIONS parameter which must be either ASYNC or NONE).

The default value is platform dependent. The COMPATIBLE.ASM and COMPATIBLE.RDBMS diskgroup attributes must be set to 11.2 or higher to set the sector size to a value other than the default value.

Note:-  Oracle ACFS does not support 4KB sector drives directly. There is a performance penalty for Oracle ACFS when using 4KB sector disk drives in 512 sector emulation mode.

Types of sector disk:-

  1. Emulation Mode:-  This mode has eight logical sectors per physical sector.The 4k block size is recommended for redo logs, a multiple of 4K is recommended for the database block size (which most systems are), 16K will be used for the control file block size by default.

If log files are created with a 512-byte size on a system using 4K emulation, a warning will be written to the alert log regarding the potential for performance degradation.

  1. Native Mode:- It has one logical sector per physical sector.  The 4k block size is mandatory for redo logs, a multiple of 4K is mandatory for the database block size 16K will be used for the control file block size by default.

To specify a 4K size when using ASM, we add a new attribute to the CREATE DISKGROUP command, which is the sector size. The two valid values for sector_size are 4096 or 512.

ASM sector size can be managed by three different tools:-

  • SQL Command line
  • The ASM Command-line utility(ASMCMD)
  • Enterprise Manager

Why do we have sector_size optional in ASM disk group?

If we do not use this clause, ASM detects the hardware sector size and uses it. The SECTOR_SIZE dis group attribute can be set only during disk group creation.ASM provides support 4KB sector dis drives without negatively affecting performance.

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