Comparison and interpretation of RDM and vmfs

On a storage area network (SAN), you can configure storage for virtual machines in two ways. One way is to use vmfs, which is a proprietary High-Performance cluster file system provided by VMWare Infrastructure (VI. For most enterprise applications, using Virtual Disks (vmdk files) on vmfs is the first choice. All available functions are supported in VI, includes Virtual Machine snapshots, vmotion, storage vmotion, and VMWare provisioned lidated backup (VCB ). Another way to configure storage is raw device mapping (original device ing, RDM ). When you perform virtual access to the following storage, it will interfere with software operations running in the virtual machine. In this case, you need to use RDM. In this example, the SAN management software usually needs to directly access the following hardware; therefore, RDM is required instead of a virtual disk. In this article, Scott Lowe, a special author of techtarget China, will discuss what RDM is and explain when to use it. Define original device ing RDM is a file on a vmfs volume. For the original physical device, RDM is like a proxy or media. We can regard RDM as a symbolic chain to the original Lun. RDM contains metadata and other information of the accessed raw physical device. Depending on the RDM configuration, you can add features such as vmotion support and Virtual Machine snapshots using the original Lun. Why use RDM instead of Virtual Disk Files? VMware takes vmfs as a priority option for enterprise applications, including file servers, Web servers, EPR, and CRM. In earlier versions of ESX Server and vmfs, RDM is usually used to minimize the cost of vmfs to improve disk performance. However, based on the comparison of VMware's use of VMFS-3, disk performance between VMFS-3 and RDM is almost the same in most cases. In most cases, if the performance obtained by using vmfs and RDM is similar, what is the other reason for using RDM instead of vmfs? Generally, this answer lies in the application requirements. SAN management software has been used as an application that requires RDM rather than Virtual Disks, thanks to direct communication between SAN management software and storage arrays. The following examples show where RDM is needed rather than a Virtual Disk: in the MSC cluster, RDM is required to provide quorum and data disks. This includes V2V clusters and p2v clusters across ESX hosts. Note that the virtual disk can be used as the "cluster-in-a-box" configuration on a single physical host. RDM is required in the solution of the application that San knows in the virtual machine. There are many examples of this situation, such as the snapmanager series applications of netapp. These applications need to communicate directly with the storage array, So Virtual Disks cannot be used. RDM must be used in npiv (n_port ID virtualization alization) solutions. Npiv uses wwpn to allow a single fiber channel HbA port to be registered using the fiber channel structure. ESX 3.5 adds npiv support, allowing ESX to behave like a "virtual HbA" in front of virtual machines ". Npiv can only be used with RDM. 　 RDM LimitationsThere are two types of RDM: Virtual compatibility mode RDM and physical compatibility mode RDM. In particular, the physical compatibility mode RDM has some important limitations: l cannot use VMware snapshots L does not support VCB, because VCB requires VMware snapshot l Physical Mode RDM cannot clone virtual machine l Physical Mode RDM in the template cannot be converted to virtual machine l if migration includes copying disk, physical Mode RDM cannot be used to migrate virtual machines l Physical Mode RDM cannot use vmotion function virtual mode RDM can solve such problems, allow the original lun as a virtual disk, it can also use features such as vmotion, snapshot, and clone. To use RDM, the virtual mode RDM is popular in most cases. For example, the virtual mode RDM can be used for V2V clusters across physical hosts. Note that physical mode RDM is required for p2v migration across physical hosts. In the VI environment, most applications and workloads use virtual disks, however, RDM-whether in virtual mode or physical mode-can help eliminate potential compatibility issues or allow applications to run in virtual state without function loss. Reprinted from: http://www.searchsv.com.cn/ShowContent_15429.htm #