Before you get started with
vSAN
, review
the key
vSAN
terms and
definitions.
Disk Group (vSAN Original Storage Architecture)
A disk group is a unit of physical storage capacity and
performance on a host and a group of physical devices that provide performance and
capacity to the vSAN
cluster. On each ESXi
host that contributes its local devices to a vSAN
cluster, devices are organized into disk groups.
Each disk group must have one flash cache device and
one or multiple capacity devices. The devices used for caching cannot be shared
across disk groups, and cannot be used for other purposes. A single caching device
must be dedicated to a single disk group. In hybrid clusters, flash devices are used
for the cache layer and magnetic disks are used for the storage capacity layer. In
an all-flash cluster, flash devices are used for both cache and capacity. For
information about creating and managing disk groups, see
Administering VMware vSAN
.
Storage Pool (vSAN Express Storage Architecture)
A storage pool is a representation of all storage devices on a host that are claimed
by vSAN
. Each host contains one storage
pool. Each device in the storage pool contributes both capacity and performance. The
number of storage devices allowed is determined by the host configuration.
Consumed Capacity
Consumed capacity is the amount of physical capacity
consumed by one or more virtual machines at any point. Many factors determine
consumed capacity, including the consumed size of your .vmdk
files, protection replicas, and so on. When calculating for cache sizing, do not
consider the capacity used for protection replicas.
Object-Based Storage
vSAN
stores and
manages data in the form of flexible data containers called objects. An object is a
logical volume that has its data and metadata distributed across the cluster. For
example, every .vmdk
is an object, as is every snapshot. When
you provision a virtual machine on a vSAN
datastore,
vSAN
creates a set of objects comprised of multiple components
for each virtual disk. It also creates the VM home namespace, which is a container
object that stores all metadata files of your virtual machine. Based on the assigned
virtual machine storage policy, vSAN
provisions
and manages each object individually, which might also involve creating a RAID
configuration for every object.
If vSAN
Express Storage Architecture
is enabled, every snapshot is not a new object. A base
.vmdk
and its snapshots are contained in one vSAN
object. Additionally, in vSAN ESA, digest is backed by vSAN objects.
When
vSAN
creates an
object for a virtual disk and determines how to distribute the object in the
cluster, it considers the following factors:
vSAN
verifies
that the virtual disk requirements are applied according to the specified
virtual machine storage policy settings.
vSAN
verifies
that the correct cluster resources are used at the time of provisioning. For
example, based on the protection policy,
vSAN
determines
how many replicas to create. The performance policy determines the amount of
flash read cache allocated for each replica and how many stripes to create for
each replica and where to place them in the cluster.
vSAN
continually monitors and reports the policy compliance status of the virtual
disk. If you find any noncompliant policy status, you must troubleshoot and
resolve the underlying problem.
When required, you
can edit VM storage policy settings. Changing the storage policy settings does
not affect virtual machine access.
vSAN
actively
throttles the storage and network resources used for reconfiguration to
minimize the impact of object reconfiguration to normal workloads. When you
change VM storage policy settings,
vSAN
might
initiate an object recreation process and subsequent resynchronization. See
vSAN Monitoring and
Troubleshooting
.
vSAN
verifies
that the required protection components, such as mirrors and witnesses, are
placed on separate hosts or fault domains. For example, to rebuild components
during a failure,
vSAN
looks for
ESXi hosts that satisfy the placement rules where protection components of
virtual machine objects must be placed on two different hosts, or across fault
domains.
vSAN
Datastore
After you enable
vSAN
on a
cluster, a single
vSAN
datastore
is created. It appears as another type of datastore in the list of datastores
that might be available, including Virtual Volume, VMFS, and NFS. A single
vSAN
datastore
can provide different service levels for each virtual machine or each virtual
disk. In
vCenter Server®, storage characteristics of the
vSAN
datastore
appear as a set of capabilities. You can reference these capabilities when
defining a storage policy for virtual machines. When you later deploy virtual
machines,
vSAN
uses this
policy to place virtual machines in the optimal manner based on the
requirements of each virtual machine. For general information about using
storage policies, see the
vSphere Storage
documentation.
A
vSAN
datastore
has specific characteristics to consider.
vSAN
provides a
single
vSAN
datastore
accessible to all hosts in the cluster, whether or not they contribute storage
to the cluster. Each host can also mount any other datastores, including
Virtual Volumes, VMFS, or NFS.
You can use Storage
vMotion to move virtual machines between
vSAN
datastores, NFS datastores, and VMFS datastores.
Only magnetic disks and
flash devices used for capacity can contribute to the datastore capacity. The
devices used for flash cache are not counted as part of the datastore.
Objects and Components
Each object is composed of a set of components,
determined by capabilities that are in use in the VM Storage Policy. For example,
with Failures to
tolerate
set to 1, vSAN
ensures
that the protection components, such as replicas and witnesses, are placed on
separate hosts in the vSAN
cluster,
where each replica is an object component. In addition, in the same policy, if the
Number of disk
stripes per object
configured to two or more, vSAN
also stripes the object across multiple capacity devices and
each stripe is considered a component of the specified object. When needed,
vSAN
might also break large objects into multiple components.
A
vSAN
datastore
contains the following object types:
VM Home Namespace
The virtual machine home directory where all
virtual machine configuration files are stored, such as .vmx
, log
files, .vmdk
files, and snapshot delta description
files.
VMDK
A virtual machine disk
or
.vmdk
file that
stores the contents of the virtual machine's hard disk drive.
VM Swap Object
Created when a virtual
machine is powered on.
Snapshot Delta VMDKs
Created when virtual machine snapshots are taken.
Such delta disks are not created for vSAN
Express Storage Architecture.
Memory object
Created when the
snapshot memory option is selected when creating or suspending a virtual
machine.
Virtual Machine
Compliance Status: Compliant and Noncompliant
A virtual machine is
considered noncompliant when one or more of its objects fail to meet the
requirements of its assigned storage policy. For example, the status might
become noncompliant when one of the mirror copies is inaccessible. If your
virtual machines are in compliance with the requirements defined in the storage
policy, the status of your virtual machines is compliant. From the
Physical
Disk Placement
tab on the
Virtual
Disks
page, you can verify the virtual machine object compliance
status. For information about troubleshooting a
vSAN
cluster,
see
vSAN Monitoring and
Troubleshooting
.
Component State:
Degraded and Absent States
vSAN
acknowledges the following failure states for components:
Degraded. A component is
Degraded when
vSAN
detects a
permanent component failure and determines that the failed component cannot
recover to its original working state. As a result,
vSAN
starts to
rebuild the degraded components immediately. This state might occur when a
component is on a failed device.
Absent. A component is
Absent when
vSAN
detects a
temporary component failure where components, including all its data, might
recover and return
vSAN
to its
original state. This state might occur when you are restarting hosts or if you
unplug a device from a
vSAN
host.
vSAN
starts to
rebuild the components in absent status after waiting for 60 minutes.
Object State: Healthy
and Unhealthy
Depending on the type and
number of failures in the cluster, an object might be in one of the following
states:
Healthy. When at least one
full RAID 1 mirror is available, or the minimum required number of data
segments are available, the object is considered healthy.
Unhealthy. An object is
considered unhealthy when no full mirror is available or the minimum required
number of data segments are unavailable for RAID 5 or RAID 6 objects. If fewer
than 50 percent of an object's votes are available, the object is unhealthy.
Multiple failures in the cluster can cause objects to become unhealthy. When
the operational status of an object is considered unhealthy, it impacts the
availability of the associated VM.
Witness
A witness is a component that contains only metadata
and does not contain any actual application data. It serves as a tiebreaker when a
decision must be made regarding the availability of the surviving datastore
components, after a potential failure. A witness consumes approximately 2 MB of
space for metadata on the vSAN
datastore
when using on-disk format 1.0, and 4 MB for on-disk format version 2.0 and later.
vSAN
maintains a
quorum by using an asymmetrical voting system where each component might have more
than one vote to decide the availability of objects. Greater than 50 percent of the
votes that make up a VM’s storage object must be accessible at all times for the
object to be considered available. When 50 percent or fewer votes are accessible to
all hosts, the object is no longer accessible to the vSAN
datastore.
Inaccessible objects can impact the availability of the associated VM.
Storage Policy-Based
Management (SPBM)
When you use vSAN
, you can
define virtual machine storage requirements, such as performance and availability,
in the form of a policy. vSAN
ensures
that the virtual machines deployed to vSAN
datastores
are assigned at least one virtual machine storage policy. When you know the storage
requirements of your virtual machines, you can define storage policies and assign
the policies to your virtual machines. If you do not apply a storage policy when
deploying virtual machines, vSAN
automatically assigns a default vSAN
policy with
Failures to
tolerate
set to 1, a single disk stripe for each object, and thin
provisioned virtual disk. For best results, define your own virtual machine storage
policies, even if the requirements of your policies are the same as those defined in
the default storage policy. For information about working with vSAN
storage policies, see
Administering VMware vSAN
.
vSphere
PowerCLI
VMware vSphere PowerCLI adds
command-line scripting support for
vSAN
, to help
you automate configuration and management tasks. vSphere PowerCLI provides a
Windows PowerShell interface to the vSphere API. PowerCLI includes cmdlets for
administering
vSAN
components. For information about using vSphere PowerCLI, see
vSphere PowerCLI
Documentation
