ASM metadata blocks

An ASM instance manages the metadata needed to make ASM files available to Oracle databases and other ASM clients. ASM metadata is stored in disk groups and organised in metadata structures. These metadata structures consist of one or more ASM metadata blocks. For example, the ASM disk header consist of a single ASM metadata block. Other structures, like the Partnership and Status Table, consist of exactly one allocation unit (AU). Some ASM metadata, like the File Directory, can span multiple AUs and will not have the predefined size; in fact, the File Directory will grow as needed and will be managed as any other ASM file.

ASM metadata block types

The following are the ASM metadata block types:

• KFBTYP_DISKHEAD - The ASM disk header - the very first block in every ASM disk. A copy of this block will be in the second last Partnership and Status Table (PST) block (in ASM version 11.1.0.7 and later). The copy of this block will also be in the very first block in Allocation Unit 11, for disk groups with COMPATIBLE.ASM=12.1 or higher.
• KFBTYP_FREESPC - The Free Space Table block.
• KFBTYP_ALLOCTBL - The Allocation Table block.
• KFBTYP_PST_META - The Partnership and Status Table (PST) block. The PST blocks 0 and 1 will be of this type.
• KFBTYP_PST_DTA - The PST blocks with the actual PST data.
• KFBTYP_PST_NONE - The PST block with no PST data. Remember that Allocation Unit 1 (AU1) on every disk is reserved for the PST, but only some disks will have the PST data.
• KFBTYP_HBEAT - The heartbeat block, in the PST.
• KFBTYP_FILEDIR - The File Directory block.
• KFBTYP_INDIRECT - The Indirect File Directory block, containing a pointer to another file directory block.
• KFBTYP_LISTHEAD - The Disk Directory block. The very first block in the ASM disk directory. The field kfdhdb.f1b1locn in the ASM disk header will point the the allocation unit whose block 0 will be of this type.
• KFBTYP_DISKDIR - The rest of the blocks in the Disk Directory will be of this type.
• KFBTYP_ACDC - The Active Change Directory (ACD) block. The very first block of the ACD will be of this type.
• KFBTYP_CHNGDIR - The blocks with the actual ACD data.
• KFBTYP_COD_BGO - The Continuing Operations Directory (COD) block for background operations data.
• KFBTYP_COD_RBO - The COD block that marks the rollback operations data.
• KFBTYP_COD_DATA - The COD block with the actual rollback operations data.
• KFBTYP_TMPLTDIR - The Template Directory block.
• KFBTYP_ALIASDIR - The Alias Directory block.
• KFBTYP_SR - The Staleness Registry block.
• KFBTYP_STALEDIR - The Staleness Directory block.
• KFBTYP_VOLUMEDIR -The ADVM Volume Directory block.
• KFBTYP_ATTRDIR -The Attributes Directory block.
• KFBTYP_USERDIR - The User Directory block.
• KFBTYP_GROUPDIR - The User Group Directory block.
• KFBTYP_USEDSPC - The Disk Used Space Directory block.
• KFBTYP_ASMSPFALS -The ASM spfile alias block.
• KFBTYP_PASWDDIR - The ASM Password Directory block.
• KFBTYP_INVALID - Not an ASM metadata block.

Note that the KFBTYP_INVALID is not an actual block type stored in ASM metadata block. Instead, ASM will return this if it encounters a block where the type is not one of the valid ASM metadata block types. For example if the ASM disk header is corrupt, say zeroed out, ASM will report it as KFBTYP_INVALID. We will also see the same when reading such block with the kfed tool.

ASM metadata block

The default ASM metadata block size is 4096 bytes. The block size will be specified in the ASM disk header field kfdhdb.blksize. Note that the ASM metadata block size has nothing to do with the database block size.

ASM metadata block header

The first 32 bytes of an ASM metadata block contains the block header (not to be confused with the ASM disk header). The block header has the following information:

• kfbh.endian - Platform endianness.
• kfbh.hard - H.A.R.D. (Hardware Assisted Resilient Data) signature.
• kfbh.type - Block type.
• kfbh.datfmt - Block data format.
• kfbh.block.blk - Location (block number). 
• kfbh.block.obj - Data type held in this block.
• kfbh.check - Block checksum.
• kfbh.fcn.base - Block change control number (base).
• kfbh.fcn.wrap - Block change control number (wrap).

The FCN is the ASM equivalent of database SCN.

The rest of the contents of an ASM metadata block will be specific to the block type. In other words, an ASM disk header block will have the disk header specific data - disk number, disk name, disk group name, etc. A file directory block will have the extent location data for a file, etc.

Conclusion

An ASM instance manages ASM metadata blocks. It creates them, updates them, calculates and updates the check sum on writes, reads and verifies the check sums on reads, exchanges the blocks with other instances, etc. ASM metadata structures consist of one of more ASM metadata blocks. A tool like kfed can be used to read and modify ASM metadata blocks.