Other’s
- ASM can work for RAC and non-RAC databases
- One ASM instance on a node will service any number of instances on that node
- If using ASM for RAC, ASM must also be clustered to allow instances to update each other when file mapping changes occur
- The smallest unit of storage written to disk is called an “allocation unit” (AU) and is usually 1MB (4MB recommended for Exadata)
- Very simply, ASM is organized around storing files
- Files are divided into pieces called “extents”
- Extent sizes are typically equal to 1 AU, except in 11g where it will use variable extent sizes that can be 1, 8, or 64 AUs
- File extent locations are maintained by ASM using file extent maps.
- ASM maintains file metadata in headers on the disks rather than in a data dictionary
- The file extent maps are cached in the RDBMS shared pool; these are consulted when an RDBMS process does I/O
- ASM is very crash resilient since it uses instance / crash recovery similar to a normal RDBMS (similar to using undo and redo logging)
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