Tip Of the Day: 3-way Mirrors


Today’s tip…

 

In Windows Server 2012 Storage Spaces and Windows 8 Storage Spaces, a 2-way mirror requires at least 2 physical disks.  However, a 3-way mirror requires at least 5 physical disks.

 

The reason is that a 3-way mirror uses a quorum.  In order to keep running, the mirror space must keep over 50% of the disks functioning.  So a 3-way mirror must have at least 5 physical disks to be able to survive the loss of up to 2 physical disks.

 

A 2-way mirror does not require this type of majority vote.

Comments (5)

  1. MartinJ says:

    According to http://technet.microsoft.com/en-us/library/jj822937.aspx "•For two-way mirroring on failover clusters, you must have at least three physical disks." Which is correct?

  2. MartinJ says:

    Ah, sorry! The difference being, that "normal" Storage Spaces with 2-way mirroring needs two physical disks, while a Storage Space with 2-way mirroring for Failover Cluster needs at least three physical disks. But then, can someone explain why, there are
    different requirements for 2-way mirroring, when used for Failover Clusters?

  3. Bob Hyatt says:

    I think the 3rd disk is to maintain the mirror at full capacity even if a disk fails – in a normal mirror a single disk failure puts the mirror into a degraded state. Same for 3-way – 2 extra disks are needed to accommodate the (increasingly common) failure
    of 2 disks at once

  4. Bob Hyatt says:

    I think the 3rd disk is to maintain the mirror at full capacity even if a disk fails – in a normal mirror a single disk failure puts the mirror into a degraded state. Same for 3-way – 2 extra disks are needed to accommodate the (increasingly common) failure
    of 2 disks at once

  5. James B says:

    A three way mirror, or a mirror in a cluster needs a majority quorum on the Disk map, which is stored in its entirety on each disk. A two way mirror technically could need this, but doesn’t for accessibility reasons. three way mirrors or clusters are intrinsically
    ‘enterprisy’ and so can justify the extra complexity.