The post provides a quick overview of Oracle Automatic Storage Management (ASM) concepts and operations.
– 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).
How ASM stores the data
– 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)
ASM Disk Groups
– In ASM Storage is organized into “diskgroups” (DGs). A DG has a name like “DATA” in ASM which is visible to the RDBMS as a file beginning 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.
– 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.
– “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 10 (starting 18.104.22.168 it can be from 0 to 1024). 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.
– 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 result in lower latency.
– ASM can work for RAC and non-RAC databases.
– One ASM instance on a node will service any number of database 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.