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ASM 11g New Features - How ASM Disk Resync Works. (Doc ID 466326.1)

Last updated on FEBRUARY 24, 2022

Applies to:

Oracle Database Cloud Service - Version N/A and later
Oracle Database - Enterprise Edition - Version to [Release 11.1 to 11.2]
Oracle Database Cloud Schema Service - Version N/A and later
Gen 1 Exadata Cloud at Customer (Oracle Exadata Database Cloud Machine) - Version N/A and later
Oracle Database Exadata Express Cloud Service - Version N/A and later
Information in this document applies to any platform.
Oracle Server Enterprise Edition - Version: to


11g introduces new Scalability and Performance improvements for ASM, this is the case of ASM Fast Disk Resync Feature which quickly resynchronizes ASM disks within a disk group after transient disk path failures as long as the disk drive media is not corrupted. Any failures that render a failure group temporarily unavailable are considered transient failures. Disk path malfunctions, such as cable disconnections, host bus adapter or controller failures, or disk power supply interruptions, can cause transient failures. The duration of a fast mirror resync depends on the duration of the outage. The duration of a resynchronization is typically much shorter than the amount of time required to completely rebuild an entire ASM disk group.


ASM Fast Disk Resync Overview

1) When we take a disk offline in case the disk is corrupted or database is not able to read or write from the disk. In case of Oracle database 10g, oracle engine use to balance the other disks with the content of offline disk. This process was a relatively costly operation, and could take hours to complete, even if the disk failure was only a transient failure.

2) Oracle Database 11g introduces the ASM Fast Mirror Resync feature that significantly reduces the time required to resynchronize a transient failure of a disk. When a disk goes off line oracle engine doesn’t balance other disk, instead ASM tracks the allocation units that are modified during the outage. The content present in the failed disk is tracked by other disks and any modification that is made to the content of failed disk is actually made in other available disks. Once we get the disk back and attach it, the data belonging to this disk and which got modified during that time will get resynchronized back again. This avoids the heavy re-balancing activity.

3) ASM fast disk resync significantly reduces the time required to resynchronize a transient failure of a disk. When a disk goes offline following a transient failure, ASM tracks the extents that are modified during the outage. When the transient failure is repaired, ASM can quickly resynchronize only the ASM disk extents that have been affected during the outage.

4) This feature assumes that the content of the affected ASM disks has not been damaged or modified.

5) When an ASM disk path fails, the ASM disk is taken offline but not dropped if you have set the DISK_REPAIR_TIME attribute for the corresponding disk group. The setting for this attribute determines the duration of a disk outage that ASM tolerates while still being able to resynchronize after you complete the repair.

Note: The tracking mechanism uses one bit for each modified allocation unit. This ensures that the tracking mechanism very efficient.


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In this Document
 ASM Fast Disk Resync Overview
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