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With every consolidation effort, the consolidation platforms must meet a new set of business challenges that are unique to virtual infrastructures. When considering the acquisition of a storage system, it’s important to understand the impacts on disk I/O performance, data protection, and storage utilization. To begin with, a storage system must at a minimum provide the aggregated disk I/O performance of the combined distributed platforms being consolidated. Virtual infrastructures can apply a significant I/O load on disk subsystems. This load is a result of the VMware default storage design. With VMware VMFS datastore, multiple virtual disks (or VMDK files) are stored, which means that multiple virtual machines are concurrently accessing the file system. VMFS datastores are notorious for being extremely random in their read and write requirements. Failure to provide a robust storage system also has a negative impact in areas outside of serving VM data requests. These negative impacts may be experienced in areas such as backing up VM data to tape.
In addition, a consolidation platform needs to provide a high level of availability, because the business impact of a failure is magnified in direct proportion to the consolidation factor. Consider a common scenario where a department has 20 servers. To protect the data stored on each server, RAID 5 has been implemented. One of these servers incurs a disk drive failure, and during the RAID rebuild a media error is found on one of the surviving drives. This server’s RAID rebuild process fails and data is lost, requiring this data to be restored from a tapebased backup set. This process usually takes a significant amount of time and effort.
Suppose that you have deployed the same 20 servers as virtual machines in a virtual infrastructure. The VMs store their data on a shared storage platform, and the data is protected with RAID 5. The impact of the same failure just described would be 20 times greater in magnitude, because now all 20 virtual machines have lost data that must be restored.
The cost of data protection should be considered in two ways. First there is the acquisition cost of the RAID level being implemented; specifically, how many additional hard drives are required to provide fault tolerance. Second, this cost must be measured against the cost of impact to business operations if data is lost. The following paragraphs consider both forms of this question.
The previous paragraphs illustrate the potentially negative side of any consolidation effort if the consolidating platform is not more reliable than the original distributed platform. It is with this understanding that many administrators seek to deploy a form of data protection that is more resilient than what their physical servers were deployed with (typically, RAID 5). Cost, performance, and storage utilization are also considerations when searching for the appropriate level of data protection for a virtual infrastructure.
Many administrators consider RAID 10 (RAID 1+0), which provides data protection against a double disk failure and (more likely) protects against encountering a media error during the RAID reconstruction process. RAID 10 is a nested RAID technology that stripes data over pairs of RAID 1 mirrors. RAID 10 is considered to be one of the highest performing forms of RAID technologies, because it 10 does not compute parity information when committing data writes. Even with the value of its data protection and high performance, there is a significant cost to RAID 10, because this technology requires an additional 100% overhead of physical disk storage (Nx2). This high cost is counter to a consolidation effort; the use of RAID 10 immediately decreases overall storage virtualization by 50%.
In considering emerging technologies to provide fault tolerance that is on a par with RAID 10, administrators may consider implementing RAID 6 or RAID 50 (RAID 5+0). Both technologies are extensions to RAID 5, which stripes data and parity information across a set of disks, providing fault tolerance in the event of a single failed disk drive. RAID 6 extends the data protection of RAID 5 by writing a second set of parity data. RAID 6 provides high storage utilization, because it requires only a single drive beyond RAID 5 (N+2).
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