Dell AX4-5 Specifications download pdf (Page 26)

Raw device mapping (RDM)

VMware ESX 2.5 introduced a new technology called raw device mapping (RDM); this is also called a

mapped raw LUN when assigned to a virtual machine. This technology has a SCSI pass-through mode that

allows virtual machines to pass SCSI commands directly to the physical hardware. Utilities like admsnap

and admhost, when installed on virtual machines, can directly access the virtual disk when the virtual

disk is in physical compatibility mode. In virtual compatibility mode, a raw device mapping volume looks

like a virtual disk in a VMFS volume. This streamlines the development process by providing advance file

locking data protection and VMware snapshots. In RDM virtual compatibility mode certain advanced

storage-based technologies, such as expanding an RDM volume at the virtual machine level using

metaLUNs, do not work.

Using a raw CLARiiON LUN, a user can create a raw device mapping volume by creating a mapping file

on a VMFS volume. This mapping file, which contains a .vmdk extension, points to the raw device, as

shown in

Figure 16. The mapping file is created when the raw device is ready to be assigned to a virtual

machine. The entire CLARiiON LUN is presented to an individual virtual machine. The virtual machine

opens the mapping file information from the VMFS volume and can directly access the raw device

mappings volume for reading and writing.

For more information on configuring VMFS and raw device mapping volumes, refer to the ESX 4.0 Basic

System Administration guide.

LUN layout recommendations

This section discusses some of the best practices for optimal capacity when designing and implementing

the LUN layout for VMware ESX servers connected to CLARiiON storage systems.

OS images and application data images of virtual machines can reside on CLARiiON LUNs. Since VMFS

is a clustered file system, when LUNs are configured as VMFS volumes, many ESX servers can share

different virtual disks on the same LUN (VMFS) volume. Hence, the number of virtual machines images

installed on that particular LUN, and the workload on those virtual machines and the ESX servers that are

accessing the LUN, will dictate the number of spindles that need to be assigned to that particular LUN

(VMFS volume).

We recommend that you use striped metaLUNs to distribute the load across different RAID groups when

booting a number of OS images on a given LUN (VMFS volume), since most users assign larger LUNs to

their VMware ESX servers. When installing a guest operating on a CLARiiON LUN, configure the LUN to

use RAID 1/0 or RAID 5. Choose RAID 1/0 instead of RAID 5 to reduce rebuild times if there is a disk

failure, and to reduce required drive counts when workloads are performance-bound as opposed to

capacity-limited. Choose RAID 5 to provide the best efficiency of RAW storage for VMs that are capacity-

bound as opposed to performance-limited.

For I/O-intensive application data volumes, it is best to separate OS images from application data. In this

case, EMC recommends that you use either RDM or a single virtual disk configured on a VMFS volume;

since they are dedicated to only one virtual machine (that is, the entire LUN is presented to the virtual

machine), replication and backup of applications are almost similar to that of a physical server. The

management complexity might increase if multiple RDM volumes are created on the servers. A mix of

VMFS and raw device mapping volumes are allowed on an ESX server. However, note that ESX Server

2.x has a limit of 128 SCSI disks. This limit includes both local devices and SAN LUNs. With VMware

ESX 4.0, 3.x/ESXi, the limit is increased to 256 SCSI disks.

Also, because of the use of VMware redo logs, EMC recommends that you use separate disks for test and

development applications, virtual machine templates (because of sequential I/O intensity), and production

LUNs. Virtual machine templates, ISO images, and archived VMs are good candidates for SATA/ATA

drives.

VMware ESX 4.0/3.x/ESXi provide performance and reliability improvements where a single swap file is

available for each virtual machine. These swap files and NVRAM files for a given VM can reside on a

EMC CLARiiON Integration with

VMware ESX Server

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