What is Parallel Rebuild / Fast Rebuild?

             Windows Server 2012 R2 Storage Spaces includes the ability to automatically rebuild storage spaces using free space in a storage pool instead of using hot spares. If a physical disk fails, Storage Spaces will regenerate the data that belongs to the failed physical disk in parallel. During parallel regeneration, a single disk in the pool either serves as a source of data or the target of data; during regeneration, Storage Spaces maximizes peak sequential throughput to complete the regeneration quickly no user action is necessary, as a newly created storage space will use the new policy.

            The parallel rebuild process is designed to return the system to a resilient state as quickly as possible. To do so, it uses all the drives in the pool, which has the consequence of increasing the I/O load on the system in order to return the system to resilient state. However, for certain workloads, this may not be a desirable tradeoff. Certain deployments may choose to prioritize servicing production I/O over returning the system to a resilient state.

v How to enable ?

To set repair policy to Parallel

set-storagepool pool1 -RepairPolicy Parallel

To check repair policy

Get-storagepool pool1 | ft RepairPolicy

Repair Policy:

        Specifies how the operating system proceeds with repairing virtual disks in the specified storage

pool. The acceptable values for this parameter are:

  • Sequential Repair processes one allocation slab at a time. Specifying this value results in longer repair times, but smaller impact on I/O load.
  • Parallel Repair processes as many allocation slabs as it can in parallel. Specifying this value results in the shortest repair time, but significantly impacts I/O load.

To set RetireMissingPhysicalDisks to Alway

Set-storagepool pool1 -RetireMissingPhysicalDisks Always

To check RetireMissingPhysicalDisks

Get-storagepool pool1 ft RetireMissingPhysicaldisks


       Specifies when Windows should set the Usage property of physical disks missing from a storage

pool to Retired. The acceptable values for this parameter are:

  • Auto This is the default setting for storage pools. When set to Auto, Windows retires missing disks, but doesn’t automatically rebuild affected virtual disks unless there are physical disks whose Usage value is set to HotSpare, in which case Windows rebuilds the virtual disks five minutes after the failed write operation.
  • Always This is the recommended setting when using free pool space to rebuild storage spaces instead of using hot‐spare disks. When set to Always, Windows retires missing physical disks and automatically rebuilds affected virtual disks five minutes after the failed write operation.
  • Never When set to Never, Windows never retires missing physical disks.

v Parallel Rebuild Test and verified

        1. Configure storage pool and enable RetireMissingPhysicalDisks to Always.

        2. Create 4 virtual disks and ensure OperationalStatus is “ok”.

       3. Pull out a physical disk as failed disk or missing disk then OperationalStatus  will become “Incomplete”. (Event ID               203, 205)

     4. Once any IO through the VDs, the VD stats will become degraded. Once  Windows detected any VD “degraded”,                  Windows will active auto parallel rebuild. OperationalStatus become “in service”. (Event ID 304)

    5. Once VDs fixed then OperationalStatus become “ok”.

v What kinds of Event ID you may meet during Parallel Rebuild ?

1. Event ID 203 / 205 ‐Failed disk or missing disk



Event ID 203

Event ID 205

2. Event ID – Windows attempt to repair VDs

                                                                                                                Event ID 304

3. Event ID 305 – VD is now healthy

More detail please download the file, thanks.