ASM key concepts

The following points describe key concepts of ASM; for more information refer to the resources at the end of this note.  

• ASM exists to manage file storage for the RDBMS
• ASM does NOT perform I/O on behalf of the RDBMS
• I/O is performed by the RDBMS processes as it does with other storage types
• Thus, ASM is not an intermediary for I/O (would be a bottleneck)
• I/O can occur synchronously or asynchronously depending on the value of the DISK_ASYNCH_IO parameter
• Disks are RAW devices to ASM
• Files that can be stored in ASM: typical database data files, control files, redologs, archivelogs, flashback logs, spfiles,
  RMAN backups and incremental tracking bitmaps, datapump dumpsets.
• In 11gR2, ASM has been extended to allow storing any kind of file using Oracle ACFS capability (it appears as another filesystem to clients). Note that database files are not supported within ACFS

• ASM Basics
• 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 11.1 and 11.2 where it will use variable extent sizes that can be 1, 8, 64 AUs  or   1, 4, 16 AUs respectively
• 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)
  
  
• Storage is organized into "diskgroups" (DGs)
• A DG has a name like "DATA" in ASM which is visible to the RDBMS as a file begining with  "+DATA"; when tablespaces are created, they refer to a DG for storage such as "+DATA/.../..."
• Beneath a diskgroup are one or more failure groups (FGs)
• FGs are defined over a set of "disks"
• "Disks" can be based on raw physical volumes, a disk partition, a LUN presenting a disk array, or even an LVM or NAS device
• FGs should have disks defined that have a common failure component, otherwise ASM redundancy will not be effective
  
  
• High availability
• ASM can perform mirroring to recover from device failures
• You have a choice of EXTERNAL, NORMAL, OR HIGH redundancy mirroring
  EXTERNAL means allow the underlying physical disk array do the mirroring
  NORMAL means ASM will create one additional copy of an extent for redundancy   
  HIGH means ASM will create two additional copies of an extent for redundancy
• Mirroring is implemented via "failure groups" and extent partnering; ASM can tolerate the complete loss of all disks in a failure group when NORMAL or HIGH redundancy is implemented

• FG mirroring implementation
• Mirroring is not implemented like RAID 1 arrays (where a disk is partnered with another disk)
• Mirroring occurs at the file extent level and these extents are distributed among several disks known as "partners"
• Partner disks will reside in one or more separate failure groups (otherwise mirror copies would be vulnerable)
• ASM automatically choses partners and limits the number of them to less than 10 (varies by RDBMS version) in order to contain the overall impact of multiple disk failures
• If a disk fails, then ASM updates its extent mapping such that reads will now occur on the surviving partners
• This is one example when ASM and the RDBMS communicate with each other
• The failed disk is offlined
• In 11g, while the disk is offline, any changes to files are tracked so that those changes can be reapplied if the disk is brought online within a period of time (3.6 hours by default value of DISK_REPAIR_TIME). This could happen in cases of a bad controller or similar problem rather than the failure of the disk itself
• The tracking occurs via a bitmap of changed file extents; the bitmaps tell ASM which extents need to be copied back to the repaired disk from the partner
• This is called "fast mirror resync"
• In 10g, the disk is offlined and dropped - there is no repair time grace period before dropping.
• If the disk cannot be onlined, it must be dropped. A new disk will be installed and ASM will copy the data back via a "rebalancing" operation.  This happens automatically in the background
  
  
• Rebalancing
• "Rebalancing" is the process of moving file extents onto or off of disks for the purpose of evenly distributing the I/O load of the diskgroup
• It occurs asynchronously in the background and can be monitored
• In a clustered environment, rebalancing for a disk group is done within a single ASM instance only and cannot be distributed across multiple cluster node to speed it up
• ASM will automatically rebalance data on disks when disks are added or removed
• The speed and effort placed on rebalancing can be controlled via a POWER LIMIT setting
• POWER LIMIT controls the number of background processes involved in the rebalancing effort and is limited to 11.  Level 0 means no rebalancing will occur
• I/O performance is impacted during rebalancing, but the amount of impact varies on which disks are being rebalanced and how much they are part of the I/O workload.  The default power limit was chosen so as not to impact application performance
  
  
• Performance
• ASM will maximize the available bandwidth of disks by striping file extents across all disks in a DG
• Two stripe widths are available: coarse which has a stripe size of 1 AU, and fine with stripe size of 128K
• Fine striping still uses normally-sized file extents, but the striping occurs in small pieces across these extents in a round-robin fashion
• ASM does not read from alternating mirror copies since disks contain primary and mirror extents and I/O is already balanced
• By default the RDBMS will read from a primary extent; in 11.1 this can be changed via the PREFERRED_READ_FAILURE_GROUP parameter setting for cases where reading extents from a local node results in lower latency.  Note: This is a special case applicable to "stretch clusters" and not applicable in the general usage of ASM
  
  
• Miscellaneous
• 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
• In 11.2 onwards, ASM is installed in a grid home along with the clusterware as opposed to an RDBMS home in prior versions.