Author: mac

When an ASM disk becomes unavailable, ASM drops it from the disk group, right? Well, that depends on the ASM version and on the disk group redundancy. An external redundancy disk group would be simply dismounted, so we will focus on the normal and high redundancy disk groups. The disk would simply be dropped in ASM version 10g. Starting from…

ASM disk header is probably the best known piece of ASM metadata. Chances are you learned about it when it was damaged or lost and hopefully Oracle Support was able to get you up and running. In this post I will try to explain why ASM disk header is important and what it contains. Block zero ASM disks…

Here are the ASM tools I recommend you get familiar with. asmcmd – command line interface to ASM When ASM was released the asmcmd was useless. Today, in version 11gR2, the asmcmd is a truly versatile and useful tool. ASMCA – ASM configuration assistant ASMCA has two flavours – the pretty one, with the graphical user interface and the…

When ASM creates a file, e.g. on a request from an RDBMS instance, it allocates space in extents. Once the file is created, ASM passes the extent map to the RDBMS instance that can then access the file without involving ASM. If a file extent needs to be relocated (e.g. due to a disk group rebalance), ASM would…

ASM provides data redundancy by placing mirrored extent copies on partner disks in different failgroups. Each ASM disk, in a redundant disk group of course, would have up to 8 partner disks (up to 10 partners in ASM version 10). There is no disk partnership and there are no failgroups in external redundancy disk groups. If a normal…

When using ASMLIB to manage ASM disks, the device path info is not displayed in gv$asm_disk.path. If you are using ASMLIB Support Tools 2.1. and later (package oracleasm-support-2.1*) you can get that info by running ‘oracleasm querydisk -p’ as root: # ls -l /dev/oracleasm/disks total 0 brw-rw—- 1 grid asmadmin 8,  5 May  2 12:00…

An ASM instance manages metadata needed to make ASM files available to Oracle databases and ASM clients. ASM metadata is stored in disk groups – in metadata blocks. Some ASM metadata is at the fixed position in every ASM disk, and is referred to as physically addressed metadata. Other ASM metadata is organised in files (directories) and…

The kfed is an undocumented ASM utility that can be used to read and modify ASM metadata blocks. It is a standalone utility, independent of ASM instance, so it can be used with either mounted or dismounted disk groups. The most powerful kfed feature is its ability to fix corrupt ASM metadata. The kfed binary is present in…

Oracle ASM uses disk groups to store data files. An ASM disk group is a collection of disks managed as a unit. Within a disk group, ASM exposes a file system interface for Oracle database files. The content of files that are stored in a disk group is evenly distributed to eliminate hot spots and…

ASM Allocation Units An ASM allocation unit (AU) is the fundamental space unit within an ASM disk group. Every ASM disk is divided into allocation units. When a disk group is created, the allocation unit size can be set with the  disk group attribute AU_SIZE (in ASM versions 11.1 and later). The AU size can be 1, 2, 4,…