System information

If you decide to run the SAN management software on a virtual machine, you gain the benefits of running a
virtual machine, including failover using VMotion and VMware HA. Because of the additional level of
indirection, however, the management software might not be able to detect the SAN. This problem can be
resolved by using an RDM.
NOTE Whether a virtual machine can run management software successfully depends on the particular storage
system.
How Virtual Machines Access Data on a SAN
ESX/ESXi stores a virtual machine's disk files within a VMFS datastore that is deployed on a SAN storage
device. When virtual machine guest operating systems issue SCSI commands to their virtual disks, the
virtualization layer translates these commands to VMFS file operations.
When a virtual machine interacts with its virtual disk stored on a SAN, the following process takes place:
1 When the guest operating system in a virtual machine reads or writes to SCSI disk, it issues SCSI
commands to the virtual disk.
2 Device drivers in the virtual machine’s operating system communicate with the virtual SCSI controllers.
3 The virtual SCSI Controller forwards the command to the VMkernel.
4 The VMkernel performs the following tasks.
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Locates the file in the VMFS volume that corresponds to the guest virtual machine disk.
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Maps the requests for the blocks on the virtual disk to blocks on the appropriate physical device.
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Sends the modified I/O request from the device driver in the VMkernel to the physical HBA (host
HBA).
5 The host HBA performs the following tasks.
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Converts the request from its binary data form to the optical form required for transmission on the
fiber optic cable.
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Packages the request according to the rules of the FC protocol.
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Transmits the request to the SAN.
6 Depending on which port the HBA uses to connect to the fabric, one of the SAN switches receives the
request and routes it to the storage device that the host wants to access.
This storage device appears to be a specific disk to the host, but it might be a logical device that corresponds
to a physical device on the SAN. The switch must determine which physical device is made available to
the host for its targeted logical device.
Understanding Multipathing and Failover
To maintain a constant connection between an ESX/ESXi host and its storage, ESX/ESXi supports multipathing.
Multipathing is a technique that lets you use more than one physical path that transfers data between the host
and external storage device.
In case of a failure of any element in the SAN network, such as an adapter, switch, or cable, ESX/ESXi can
switch to another physical path, which does not use the failed component. This process of path switching to
avoid failed components is known as path failover.
In addition to path failover, multipathing provides load balancing. Load balancing is the process of distributing
I/O loads across multiple physical paths. Load balancing reduces or removes potential bottlenecks.
NOTE Virtual machine I/O might be delayed for up to sixty seconds while path failover takes place. These
delays allow the SAN to stabilize its configuration after topology changes. In general, the I/O delays might be
longer on active-passive arrays and shorter on activate-active arrays.
Fibre Channel SAN Configuration Guide
22 VMware, Inc.