Following are the factors which affect the DRS recommendations:

1- VM size and Initial Placement: When a new VM is created or a VM is powered on, DRS selects a host where this VM should be initially placed. DRS prefers the registered host as long as placement of that VM on this host will not cause cluster imbalance.

During placement of such VM’s DRS uses a worst CASE scenario because it doesn’t have historical data for that VM. DRS assumes both CPU and memory demand of this VM is equal to its configured size.

Oversized VM’s can temporarily cause cluster imbalance and can cause unnecessary migrations of active VM’s.

2- PollPeriodSec (Length of DRS INVOCATION): The default value of PollPeriodSec is 300 seconds. Range of PollPeriodSec is 60 sec to 3600 sec. shortening this period will cause increase in vCenter overhead as cluster imbalance will be calculated frequently.

INCREASING this value decreases the frequency of cluster balance calculation and can leave your cluster imbalanced for longer period of times but ALLOWS for larger number of vMotions due to long invocation interval.

3- Simultaneous vMotion: vSphere 5.1 allows 8 concurrent vMotions on a single host with 10GbE capabilities. For 1GbE, 4 concurrent vMotion can takes place. Also multi-NIC vMotion is supported in vSphere 5.1 so multiple active NICs and their combined bandwidth can be used for migration of a VM. In such environment VM’s will be migrated quickly and cluster can be balanced in less time.

4- Estimated Total Migration Time: The migration time depends on variables like source and destination host load, active memory usage of VM, link speed and available bandwidth+ latency of the physical network used by vMotion Portgroup.

Following are the factors which affect the DRS recommendations:

1- VM size and Initial Placement: When a new VM is created or a VM is powered on, DRS selects a host where this VM should be initially placed. DRS prefers the registered host as long as placement of that VM on this host will not cause cluster imbalance.

During placement of such VM’s DRS uses a worst case scenario because it doesn’t have historical data for that VM. DRS assumes both CPU and memory demand of this VM is equal to its configured size.

Oversized VM’s can temporarily cause cluster imbalance and can cause unnecessary migrations of active VM’s.

2- PollPeriodSec (Length of DRS Invocation): The default value of PollPeriodSec is 300 seconds. Range of PollPeriodSec is 60 sec to 3600 sec. shortening this period will cause increase in vCenter overhead as cluster imbalance will be calculated frequently.

Increasing this value decreases the frequency of cluster balance calculation and can leave your cluster imbalanced for longer period of times but allows for larger number of vMotions due to long invocation interval.

3- Simultaneous vMotion: vSphere 5.1 allows 8 concurrent vMotions on a single host with 10GbE capabilities. For 1GbE, 4 concurrent vMotion can takes place. Also multi-NIC vMotion is supported in vSphere 5.1 so multiple active NICs and their combined bandwidth can be used for migration of a VM. In such environment VM’s will be migrated quickly and cluster can be balanced in less time.

4- Estimated Total Migration Time: The migration time depends on variables like source and destination host load, active memory usage of VM, link speed and available bandwidth+ latency of the physical network used by vMotion Portgroup.