7.4 Traditional and Virtual Storage Provisioning Overview

Traditional Storage Provisioning

It is also known as Thick Provisioning, In Traditional storage provisioning, physical storage drives are logically grouped together on which a required RAID level is applied to form a set, called RAID set. The number of drives in the RAID set and the RAID level determine the availability, capacity, and performance of the RAID set. 
Traditional Storage Provisioning

RAID sets usually have a large capacity because they combine the total capacity of individual drives in the set. Logical units are created from the RAID sets by partitioning  the available capacity into smaller units. These units are then assigned to the hosts based on their storage requirements. Logical units are spread across all the physical drives that belong to that set. Each logical unit created from the RAID set is assigned a unique ID, called a logical unit number (LUN). 

LUNs hide the organization and composition of the RAID set from the hosts. LUNs created by traditional storage provisioning methods are also referred to as thick LUNs to distinguish them from the LUNs created by virtual provisioning methods.

Also Read: Block Based Storage Systems (SAN) Overview

When a LUN is configured and assigned to a non-virtualized host, a bus scan is required to identify the LUN. This LUN appears as a raw storage drive to the operating system. To make this drive usable, it is formatted with a file system and then the file system is mounted.
Whereas in a virtualized environment, the LUN is assigned to the hypervisor, which recognizes it as a raw storage drive. This drive is configured with the hypervisor file system, and then virtual storage drives are created on it. 

MetaLUN is a method to expand LUNs that require additional capacity or performance. A metaLUN can be created by combining two or more LUNs. A metaLUN consists of a base LUN and one or more component LUNs. MetaLUNs can be either concatenated or striped.
Concatenated expansion simply adds additional capacity to the base LUN. Concatenated expansion is quick but does not provide any performance benefit. Striped expansion restripes the base LUN’s data across the base LUN and component LUNs. In striped expansion, all LUNs must be of the same capacity and RAID level. Striped expansion provides improved performance due to the increased number of drives being striped.

All LUNs in both concatenated and striped expansion must reside on the same storage drive type either all SSDs, Fibre Channel, or all ATA.

Virtual Storage Provisioning 

It is also known as Thin Provisioning. Virtual provisioning enables creating and presenting a LUN with more capacity than is physically allocated to it on the storage system. The LUN created using virtual provisioning is called a thin LUN to distinguish it from the traditional LUN.
Virtual Storage Provisioning

Thin LUNs do not require physical storage to be completely allocated to them at the time they are created and presented to a hosts. Physicall storage is allocated to the host “on-demand” from a shared pool of physical capacity. A shared pool consists of physical storage drives. Similar to a RAID set, a shared pool supports a single RAID protection level. However, unlike a RAID set, a shared pool might contain large numbers of drives. Shared pools can be homogeneous (containing a single drive type) or heterogeneous (containing mixed drive types, such as SSD, FC, SAS, and SATA drives).

Virtual provisioning enables more efficient allocation of storage to hosts. Virtual provisioning also enables over-subscription, where more capacity is presented to the hosts than is actually available on the storage system. Both the shared pool and the thin LUN can be expanded non-disruptively as the storage requirements of the hosts grow. Multiple shared pools can be created within a storage system, and a shared pool may be shared by multiple thin LUNs.

Also Read: Factors affecting SAN performance

When a storage pool is expanded, the sudden introduction of new empty drives combined with relative full drives cause a data imbalance. This imbalance is resolved by automating a one-time data relocation, referred to as rebalancing. Storage pool rebalancing is a technique that provides the ability to automatically relocate extents on physical storage drives over the entire pool when new drives are added to the pool. Storage pool rebalancing restripes data across all the drives both existing and new drives in the storage pool. This enables spreading out the data equally on all the physical drives within the storage pool, ensuring that the used capacity of each drive is uniform across the pool. After the storage pool capacity is increased, the capacity of the existing LUNs can be expanded.

Despite the huge benefits of thin provisioning, over time thin volumes get less and less thin, which is called bloat. Without a way to remove the bloat, the thin volumes use all their possible space, making them like a thick volumes. This is where space reclamation comes to the rescue, by giving us a mechanism to keep thin volumes thin.

Space reclamation is a technology that recognizes deleted data and can release the extents that host the deleted data back to the free pool on the array, where they can be used for space for other volumes.

Limitations of Virtual Storage Provisioning

  • Thin provisioning based storage arrays allow an administrator to over-provision the array. Over-provisioning allows an array to pretend it has more capacity than it does. This allows improved efficiency in capacity utilization. However, on the downside, over-provisioning introduces the risk that the array will run out of capacity and not be able to satisfy the storage demands of attached hosts. As a result, hosts will not be able to write to new areas of volumes, which may cause hosts and applications to crash. So a careful planning and foresight is required in view of the over-provisioning.
  • Heavily sequential read workloads can suffer poor performance compared to thick volumes.
  • There is the potential for a small write performance overhead each time new space is allocated to a thin volume.
  • Over-provisioning does not simplify capacity management or storage administration. It makes it harder, and it adds pressure on the storage administrator. To gain the potential benefits, thin provisioning and over-provisioning require some good planning and foresight.
Also Read: Protecting the data for short term (Backup)

Traditional vs Virtual Storage Provisioning

  • Traditional LUNs are suited for applications that require predictable performance. Traditional LUNs provide full control for precise data placement and allow an administrator to create LUNs on different RAID groups if there is any workload contention.
  • Administrators typically allocate storage capacity based on anticipated storage requirements. This generally results in the over provisioning of storage capacity, which then leads to higher costs and lower capacity utilization. Administrators often over-provision storage to an application for various reasons such as, to avoid frequent provisioning of storage if the LUN capacity is exhausted, and to reduce disruption to application availability. Over provisioning of storage often leads to additional storage acquisition and operational costs.
  • Virtual provisioning addresses these challenges. Virtual provisioning improves storage capacity utilization and simplifies storage management. Virtual provisioning and thin LUN offer many benefits, although in some cases traditional LUN is better suited for an application. 
  • Thin LUNs are appropriate for applications that can tolerate performance variations. In some cases, performance improvement is perceived when using a thin LUN, due to striping across a large number of drives in the pool. However, when multiple thin LUNs contend for shared storage resources in a given pool, and when utilization reaches higher levels, the performance can degrade. 
  • Thin LUNs provide the best storage space efficiency and are suitable for applications where space consumption is difficult to forecast. Using thin LUNs benefits organizations in reducing power and acquisition costs and in simplifying their storage management.
Organizations that are not highly concerned about storage space efficiency may still use traditional LUNs. Both traditional and thin LUNs can coexist in the same storage system. Based on the requirement, an administrator may migrate data between thin and traditional LUNs.

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