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Data ONTAP organizes disks into RAID groups, which are collections of data and parity disks to provide parity protection. For Data ONTAP 6.5 onwards the following RAID types are supported for NetApp storage systems:

RAID4 technology: In this RAID, within each RAID group, a single disk is assigned for holding parity data, which ensures against data loss due to a single disk failure within a group.

RAID-DP™ technology (DP for double-parity): RAID-DP provides a higher level of RAID protection for Data ONTAP aggregates. Within its RAID groups, it allots one disk for holding parity data and one disk for holding double-parity data. Double-parity protection ensures against data loss due to a double disk failure within a group.

For native storage, Data ONTAP uses RAID-DP or RAID4 groups to provide parity protection. For third-party storage, Data ONTAP uses RAID0 groups to optimize performance and storage utilization. The storage arrays provide the parity protection for third-party storage. Data ONTAP RAID groups are organized into plexes, and plexes are organized into aggregates.

This test auto discovers the RAID groups in the storage system and helps the administrator figure out the following:

Measurement	Description	Measurement Unit	Interpretation
Prefailed_disks	Indicates the number of prefailed disks in this RAID group.	Number	The disks that are manually failed due to excessive error logging are termed as Prefailed disks. The contents of these disks are copied into suitable replacement disks i.e., the spare disks available in the storage system. Ideally, the value of this measure should be 0.
Replacing_disks	Indicates the number of replacing disks in this RAID group.	Number	Mismatched disks that are part of an aggregate can be replaced with a more suitable spare disk without disrupting the data service. This process uses the Rapid RAID Recovery process to copy the data from the disk being replaced to a specified spare disk. Frequently replacing the disks will lead to the system degradation which inturn may cause rapid performance degradation. Therefore the frequent replacing of the disks needs to be avoided by proper initial configuration.
Total_Physical_space	Indicates the total size of this RAID group.	MB
Total_used_space	Indicates the total amount of space used by all disks in this RAID group.	MB	Ideally, the value of this measure should be low. If this value grows close to that of the Total_Physical_space measure, then you may want to consider to add more disks to the storage system, or free space in the disks by deleting unnecessary data.
Total_used	Indicates the percent of space that is utilized across all disks in this RAID group.	Percent	A low value is desired for this measure. A value close to 100% indicates excessive disk space usage by a RAID group.
Media_scrub	Indicates the percentage of media scrubbing that is currently completed in this RAID group.	Percent	Media scrubbing is a continuous background process. The purpose of the continuous media scrub is to detect and correct media errors in order to minimize the chance of storage system disruption due to a media error while a storage system is in degraded or reconstruction mode. By default, Data ONTAP runs continuous background media scrubbing for media errors on all storage system disks. If a media error is found, Data ONTAP uses RAID to reconstruct the data and repairs the error. Due to media scrubbing process, the disk LEDs may blink on an apparently idle storage system and some CPU activity may occur even when no user workload is present.
Parity_scrub	Indicates the percentage of parity scrubbing that is currently completed in this RAID group.	Percent	The purpose of the parity scrub is to detect and correct errors in the parity disk of the RAID group. A consistent parity is required for disk reconstruction.