Fixed vs. thin provisioning

Once you have aggregated physical disks into a storage pool, you can provision storage from that pool by creating virtual disks. Storage Spaces supports two ways of provisioning virtual disks:

• Fixed provisioning The size of the virtual disk is the actual amount of physical storage space allocated from the pool.
• Thin provisioning The size of the virtual disk represents the maximum amount of physical storage space that can be allocated from the pool. No space is actually used, however, until data is stored on a volume on the virtual disk, and the amount of space used will grow or shrink as data is written to or deleted from the disk.

Note: Storage Space stores data for a volume on a virtual disk by striping the data across all physical disks in the pool. Data is interleaved in 256-MB segments called slabs. Storage Spaces then keeps track of which slab on which disk corresponds to which portion of each volume on each virtual disk provisioned from the pool.

Resiliency

Storage Spaces can be used to provide resilient storage similar (but not identical) to RAID 0 (disk striping) and RAID 1 (mirroring) that can be implemented using hardware RAID solutions. As long as your storage pool has a sufficient number of physical disks in it, you can use Storage Spaces to create virtual disks whose storage layout is any of the following three types:

• Simple The data on volumes created on this type of virtual disk is striped across all physical disks in the pool. You can use simple virtual disks to provision the maximum amount of storage from the pool, but they provide no resiliency against physical disk failure.
• Mirror The data on volumes created on this type of virtual disk is striped across all physical disks in the pool. Each segment of data is also duplicated on either two or three physical disks, as specified when the mirrored virtual disk is created, so that a copy of all data will still be available if a physical disk fails in the pool. Mirror virtual disks provide resiliency to help protect you from data loss arising from the failure of a physical disk in the pool. The degree of resiliency provided depends on the number of physical disks in the pool-for example:
  • A pool containing two physical disks can be used to create mirror virtual disks that are resilient against the failure of a single physical disk.
  • A pool containing five physical disks can be used to create mirror virtual disks that are resilient against the failure of two physical disks.
• Parity The data on volumes created on this type of virtual disk, together with parity information that can be used to facilitate automatic reconstruction of data in the event of a physical disk failure, is striped across all physical disks in the pool. Parity virtual disks also provide resiliency to help protect you from data loss arising from the failure of a physical disk in the pool, but they perform better with large sequential disk writes than with random I/O.

Note:
If your storage pool has only one physical disk in it, you will only be able to provision simple virtual disks from it. To provision mirror or parity virtual disks from a storage pool, the pool must have at least two physical disks in it.

When a physical disk fails in a pool being used to provision resilient (mirror or parity) virtual disks, Storage Spaces will continue to provide access to data stored on volumes on the virtual disk and will automatically regenerate data copies for all affected virtual disks as long as there are sufficient alternate physical disks available in the pool. When you add a physical disk to a storage pool, you have a choice of two ways to allocate the disk to the pool:

• Automatic The pool will automatically use the disk for storing data written to any volumes created on the disk.
• Hot-spare The disk will be held in reserve by the pool so that it can be used if another physical drive in the pool fails.

Note:
In the event of a simultaneous failure of multiple physical drives in a pool, you will still be able to access data stored on volumes created on resilient virtual disks in the pool as long as a simple majority (quorum) of physical disks in your pool are still healthy.