How to rebuild Geom mirror in FreeBSD

This article discusses how to rebuild a Geom mirror after a faulty drive has been replaced.

Step 1 - Identify Replaced Drive

1. Log in to your server and as root run the following command:
   
   # gmirror list
   
2. This will produce output similar to the following
   
   Geom name: gm0
   State: DEGRADED
   Components: 2
   Balance: round-robin
   Slice: 4096
   Flags: NONE
   GenID: 0
   SyncID: 3
   ID: 3594034660
   Providers:
   1. Name: mirror/gm0
      Mediasize: 159999999488 (149G)
      Sectorsize: 512
      Mode: r2w2e5
   Consumers:
   1. Name: ad4
      Mediasize: 160000000000 (149G)
      Sectorsize: 512
      Mode: r1w1e1
      State: ACTIVE
      Priority: 0
      Flags: DIRTY
      GenID: 0
      SyncID: 3
      ID: 4064102945
   

   In this example we can see the status of the mirror is degraded and the only active drive is 'ad4'.

3. To find out which drives are available and the corresponding device name run the atacontrol list command.
   # atacontrol list
   

   This will produce a list of drives with device names which are connected to your server.

   ATA channel 2:
       Master:  ad4 <WDC WD1602ABKS-18N8A0/02.03B04> SATA revision 2.x
       Slave:       no device present
   ATA channel 3:
       Master:  ad6 <WDC WD2502ABYS-02B7A0/02.03B03> SATA revision 2.x
       Slave:       no device present
   

   From this we can see that device '*ad6*' is the device name of the new drive we can use to rebuild the mirror. You should already be aware of the device name from alerts generated by your server when the drive failed; the replacement drive will show up with the same device name.

Step 2 - Rebuild Mirror

1. Now that we have verified device name of the new drive we can easily rebuild the array using the following commands:
   # gmirror forget gm0
   # gmirror insert gm0 <device>
   

   Where <device> is the device name of the new drive we determined above. In the example above <device> would be 'ad6'.

2. To verify the drive has been added to the mirror and the mirror is being rebuild run the '*gmirror list'* command.
   # gmirror list
   Geom name: gm0
   State: DEGRADED
   Components: 2
   Balance: round-robin
   Slice: 4096
   Flags: NONE
   GenID: 0
   SyncID: 3
   ID: 3594034660
   Providers:
   1. Name: mirror/gm0
      Mediasize: 159999999488 (149G)
      Sectorsize: 512
      Mode: r3w2e5
   Consumers:
   1. Name: ad4
      Mediasize: 160000000000 (149G)
      Sectorsize: 512
      Mode: r1w1e1
      State: ACTIVE
      Priority: 0
      Flags: NONE
      GenID: 0
      SyncID: 3
      ID: 4064102945
   2. Name: ad6
      Mediasize: 251059544064 (234G)
      Sectorsize: 512
      Mode: r1w1e1
      State: SYNCHRONIZING
      Priority: 0
      Flags: DIRTY, SYNCHRONIZING
      GenID: 0
      SyncID: 3
      Synchronized: 3%
      ID: 2247886846
   

   We can see here the new drive has been added and the mirror is being rebuilt (3% complete). Rebuild times can take anywhere from 20 minutes to a few hours depending on the size of the drives and the amount of application I/O on your system.
   
   The first few lines show that overall state of the mirror, once the state changes from 'DEGRADED' to 'COMPLETE' the rebuild is complete.