5.8

Table Of Contents
See the VMware Virtual SAN Design and Sizing Guide at
http://www.vmware.com/products/virtual-san/resources.html for explanations of objects, components,
mirrors, witnesses, and Virtual SAN storage policies.
If you enable multiple point-in-time (PIT) snapshots, you must make allowances for the additional
components that each snapshot creates in the Virtual SAN storage, based on the number of disks per
virtual machine, the size of the disks, the number of PIT snapshots to retain, and the number of failures to
tolerate. When retaining PIT snapshots and using Virtual SAN storage, you must calculate the number of
extra components that you require for each virtual machine:
Number of disks x number of PIT snapshots x number of mirror and witness components
Examples of using this formula demonstrate that retaining PIT snapshots rapidly increases the number of
components in the Virtual SAN storage for every virtual machine that you configure for
vSphere Replication:
n
You have a virtual machine with two 256 GB disks for which you retain 10 MPIT snapshots, and you
set the default Virtual SAN storage policy:
n
2 (number of disks) x 10 (number of PIT snapshots) x 3 (2 mirror components + 1 witness) = 60
components for this one virtual machine.
n
You have a virtual machine with two 512 GB disks for which you retain 10 PIT snapshots, and you set
the default Virtual SAN storage policy:
n
2 (number of disks) x 10 (number of PIT snapshots) x 5 (4 mirror components of 256 GB each + 1
witness) = 100 components for this one virtual machine.
The number of PIT snapshots that you retain can increase I/O latency on the Virtual SAN storage.
Replicating Virtual Machines Using Replication Seeds
You can use replication seeds if a duplicate file is found for the virtual machine on the destination
datastore. vSphere Replication compares differences and replicates only the changed blocks.
To avoid network bandwidth consumption for the amount of data that has to be replicated on initial full
synchronization, vSphere Replication allows you to copy your virtual disk files to the remote datacenter
and point those as replication seeds during configuring replication. vSphere Replication compares the
differences and replicates only the changed blocks.
When configuring replication for a virtual machine, vSphere Replication looks for a disk with the same
filename in the target datastore. If a file with the same name exists, vSphere Replication prompts you with
a warning and offers you the option to use the target disk as a seed for replication. If you accept the
option, vSphere Replication compares the differences and replicates only the changed blocks after the
virtual machine replication is fully configured and enabled. If you do not accept the prompt, then you must
change the target location for your replication.
Note The source virtual machine must be powered off before downloading the vmdk files that are used
as seeds for the replication.
VMware vSphere Replication Administration
VMware, Inc. 56