Information Technology Reference
In-Depth Information
VMFS-5 uses a more efi cient sub-block allocation size of only 8 KB, compared to 64 KB for
VMFS-3.
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VMFS-5 lets you create virtual-mode RDMs for devices up to 62 TB in size. (VMFS-3 limits
RDMs to 2 TB in size. We'll cover RDMs later in the section “Working with Raw Device
Mappings.”)
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Even better than the improvements in VMFS-5 is the fact that you can upgrade VMFS-3
datastores to VMFS-5 in place and online—without any disruption to the VMs running on that
datastore. You're also not required to upgrade VMFS-3 datastores to VMFS-5, which further
simplii es the migration from earlier versions.
Later in this chapter in the section “Working with VMFS Datastores,” we'll provide more
details on how to create, expand, delete, and upgrade VMFS datastores.
Closely related to VMFS is the idea of multipathing, a topic that we will discuss in the next
section.
Reviewing Multipathing
Multipathing
is the term used to describe how a host, such as an ESXi host, manages storage
devices that have multiple ways (or paths) to access them. Multipathing is extremely common in
Fibre Channel and FCoE environments and is also found in iSCSI environments. We won't go so
far as to say that multipathing is strictly for block-based storage environments, but we will say
that multipathing for NFS is generally handled much differently than for block storage.
In vSphere 4, VMware and VMware technology partners spent considerable effort overhaul-
ing how the elements of the vSphere storage stack that deal with multipathing work. This archi-
tecture, known as the Pluggable Storage Architecture (PSA), is still present in vSphere 5.5 as
well. Figure 6.18 shows an overview of the PSA.
Figure 6.18
vSphere's
Pluggable Storage
Architecture is
highly modular and
extensible.
Application
Application
Operating system
Operating system
VMkernel storage stack
Emulation and file system
Pluggable
Storage
Architecture
SATP
PSP
NMP
MPP
Emulation and drivers
HBA 1
HBA 2
One of the key goals in the development of the PSA was to make vSphere multipathing much
more l exible. Pre-vSphere 4 versions of VMware ESX/ESXi had a rigid set of lists that deter-
mined failover policy and multipathing policy, and this architecture was updated only with
