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Volume creation and sizing
38 Dell EMC SC Series: Best Practices with VMware vSphere | 2060-M-BP-V
8 Volume creation and sizing
Administrators are tasked with complex decisions such as determining the best volume size, number of virtual
machines per datastore, and file system versions for their environment.
8.1 Volume sizing and the 64 TB limit
Although the maximum size of a volume that can be presented to ESXi is 64 TB, the general recommendation
is to start with smaller and more manageable initial datastore sizes and expand them as needed. Remember
that a datastore can easily be expanded to a larger size later, so it is prudent to start with datastore sizes in
the 500–750 GB range. This is based on the consideration that a 750 GB datastore will accommodate
approximately 15 x 40 GB virtual disks. This will leave a small amount of overhead for virtual machine
configuration files, logs, snapshots (replays), and memory swap, keeping the datastore performing
adequately.
The largest single extent 64 TB VMFS-5 volume size is (64 x 1,024 x 1,024 x 1,024 x 1,024 =
70,368,744,177,664) bytes. For any volumes larger than this size, VMware will not consume the additional
space.
Note: These sizing recommendations are provided to limit the number of virtual machines on each datastore
and to keep performance manageable, not for capacity reasons. If there is a virtual machine that requires a
large virtual disk, creating a large datastore to accommodate the VM is acceptable.
Note: Within certain versions of ESXi 5.x, hosts may have maximum addressable storage limits of 4 TB, 8
TB, or 60 TB due to the VMFS heap size. Read the following VMware KB article for more information:
http://kb.vmware.com/kb/1004424.
8.2 Virtual machines per datastore
There are no steadfast rules for the number of virtual machines that should be placed on a datastore. Due to
the scalability enhancements of VMFS, a good conservative approach is to place anywhere between 15 to 25
virtual machines on each.
The reason behind limiting virtual machines and VMDK files per datastore is due to potential I/O contention,
queue depth contention, or other conflicts that may degrade system performance. For the same reason, it is
advisable to limit datastore sizes to 500 to 750 GB. Size helps limit the total number of virtual machines that
can be placed on each datastore.
The art to virtual-machine placement revolves around analyzing the typical disk I/O patterns for each of the
virtual machines and placing them accordingly. In other words, the sweet spot of how many virtual machines
can be put on each datastore is dependent on the disk load. For example, sometimes the appropriate number
for high-I/O-load virtual machines may be less than five, while the number of virtual machines with low-I/O
disk requirements may be 25 or more.
The appropriate number of virtual machines that can be put onto each datastore is subjective and dependent
on the environment. A good recommendation is to start with 15 to 25 virtual machines and increase or
decrease the number of virtual machines on each datastore as needed. Moving virtual machines between
datastores can even be done nondisruptively when licensed to use the VMware Storage vMotion feature.