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Windows Server 2012 : Deploying Storage Spaces (part 2) - Understanding Storage Spaces - Fixed vs. thin provisioning

3/6/2014 3:19:07 AM

1.2 Fixed vs. thin provisioning

Once you’ve 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.

The difference between these two types of provisioning can be explained using the following example. Figure 1 shows a storage pool named Storage Pool 1 that has been created from three 2-TB physical hard drives. If you use fixed provisioning to create virtual disks from this pool, you could create, for example, one virtual disk that is 4 TBs in size and a second virtual disk that is 2 TBs in size, for a total of 6 TBs, which equals the physical storage space available in the pool. At this point, because all available storage in the pool has been provisioned, no new virtual disks can be created unless additional physical disks are added to the pool.

Creating virtual disks from a storage pool using fixed provisioning.
Figure 1. Creating virtual disks from a storage pool using fixed provisioning.

By contrast, Figure 2 shows thin provisioning being used to create virtual disks from the same storage pool of physical disks. Two virtual disks have been created: one 20 TBs in size and the other 10 TBs in size, for a total of 30 TBs. You could now create volumes on these virtual disks and use the volumes to store data. For example, three volumes of sizes 10 TBs, 5 TBs, and 5 TBs could be created on the 20-TB virtual disk, while a single 10-TB volume could be created on the 10-TB virtual disk. If you discover that the volumes you created are insufficient to meet the needs of your environment, you could perform either of the following actions:

  • Create additional thinly provisioned virtual disks from the same storage pool.

  • Extend one (or more) of your existing virtual disks to create more room on it, and then create additional volumes on the extended disk (or disks).

Creating virtual disks from a storage pool using thin provisioning.
Figure 2. Creating virtual disks from a storage pool using thin provisioning.

How can 6 TBs of physical storage space be used to create 30 TBs of NTFS-formatted volumes for storing data on? With fixed provisioning, this is not possible. However, with thin provisioning, the data volumes use space on the physical disks only when you store some data on the volumes.

What happens, then, if you begin copying large amounts of data onto the two volumes shown in Figure 2? Once you begin to approach the actual available physical capacity of the storage pool, a notification message will be displayed to alert you to the situation. At that point, you can either stop copying data to the volumes or add more physical disks to your storage pool.

1.3 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

Requirement for provisioning resilient virtual disks

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

Quorum

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.

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