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Storage > Aggregates : Understanding aggregates : Considerations for using mirrored aggregates

Considerations for using mirrored aggregates
If you want to use mirrored aggregates, you can either create a new aggregate with two mirrored plexes, or add a plex to an existing aggregate.
Note: A mirrored aggregate can have only two plexes.
The rules for the selection of disks or array LUNs, for using as mirrored aggregates, are as follows:
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If there is no equivalent-sized disk, Data ONTAP uses a larger-capacity disk, and limits the size to make it identically sized.
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Data ONTAP names the plexes of the mirrored aggregate.
Note: When creating an aggregate, Data ONTAP selects disks from the plex which has the most available disks. You can override this selection policy by specifying the disks to use.