VMware ESXi Host Memory Management, Monitoring, Alert Notification – Part 1

When it comes VMware memory monitoring – two items to monitor (i)ESXi host memory (ii)VM memory. There are bunch of memory related terminologies and calculations here in this space. I am discussing host memory monitoring here –

-understand physical memory usage monitoring
-what is the right memory counter to monitor & alert notification for esxi host
-what is the right gauge of memory monitoring & alert notification for esxi host

Will also setup Nagios check plugin to monitor the above with performance data for graph (Part 2).

Before moving forward; let’s have a look into Mem.MinFreePct function. This function manage how much host memory should be kept free and when the hypervisor should kick-off advanced memory reclamation techniques such as ballooning, compression, swapping.

(Configuration> Advanced Settings>Mem)

Based on free host memory & reclamation techniques – there are four (04) different states of host memory utilization;

State Name Mem Reclamation Technique Good or Bad Note
High At this state “Transparent Page Sharing” is will be always running. This is default behaviour. Good – this is normal This is defined by Mem.MinFreePct function. Don’t disable TPS – not recommended.
Soft At this state host will activate memory ballooning. Not good enough This is 64% of Mem.MinFreePct. This means physical memory near to max out.  If host unable to go back to previous state itself – take necessary action to free up more mem.
Hard At this state host will start doing memory compression and hypervisor level swapping. Bad – memory under stress This is 32% of Mem.MinFreePct. Need to free up memory by migrating VMs to other hosts or upgrade memory.
Low At this state host will no more serve any page to VMs. Very Bad – fix it ASAP This is 16% of Mem.MinFreePct. This protects host VMkernel layer from Purple Screen of Death.

Prior to ESXi-5.x this (high state) was set to 6% by default – this means host system will always keep 6% of total physical memory free before activate advanced memory reclamation technique; let’s say an ESXi-4.x host with 64GB memory will be required at least 3.84GB free to be in the High state (normal).

Starting from ESXi-5.x this calculation is no more 6% by default – because high memory servers (512GB/768GB) are becoming common these days; 6% of 512GB is 30.72GB its huge free memory.

The new calculation is following –

Free Memory Threshold Range Calculation Note
6% First 0GB to 4 GB 6% of 4GB
4% Starting from 4GB to 12GB (12-4=8) 4% of 8GB
2% Starting from 12GB to 28GB (28-12=16) 4% of 16GB
1% Remaining memory i.e. 36GB if total size is 64GB (64-28=36)
i.e. 68GB if total size is 96GB (96-28=68)

Based on above – on a system with 128GB memory, the min free memory required to be in “high state” calculation is following –

i. 6% of first 4GB – this is 245.76MB (first 0-4GB)
ii. 4% of 8GB – this is 327.68MB (0-4GB|4-12GB)
iii. 2% of 16GB – this is 327.68MB (0-4GB|4-12GB|12-28GB)
iv. 1% of 100GB – this is 1024MB (0-4|4-12|12-28|28-128GB)
v. Total is 1925.12MB (245.76+327.68+327.68+1024).


Based on the above we can setup monitoring & alert notification for a 128GB host as following –

Mem State Min Free Mem Monitoring Action Calculation
High 1925.12MB No action required Based on above
Soft 1232.0768MB Warning alert 64% of Mem.MinFreePct
Hard 616.384MB Critical alert 32% of Mem.MinFreePct
Low 308.0192MB Critical alert 16% of Mem.MinFreePct

Also at “Hard” state – memory performance measurement counter “Swap used” will be greater than 0. This condition also should trigger alarm.


(esxtop – memory high state)



Adding a new disk ONLINE to a Linux VM running on VMware (no reboot required)

Adding a new disk ONLINE on a virtual Linux server is easy as adding disk to a Windows 2008/2012 Server online! No reboot required.

Make sure the following software already installed on your Linux VM.

-VMware Tools (for other hypervisor install the guest plugin on the VM)



In this example I used RedHat/CentOS running on VMware.

Technical procedures are following –

Before we began let’s see how many disks are currently provisioned on the Linux VM. To do this execute “#lsscsi” command; screenshot –

In this example the server currently have three disks (03) installed.


Now add a new disk to the VM through vSphere client. Execute the same “#lsscsi” command – the newly disk will not appear!

To get this newly added disk recognized by the Linux system we need to do “rescan SCSI bus”. Usually “SCSI bus” rescan happen every time when the machine gets rebooted – however this time we don’t want to reboot the system!

Execute the following command to rescan “scsi bus” –

# /usr/bin/rescan-scsi-bus.sh -l

(this script is a part of sg3_utils)

You should be able to see the newly added disk on the command output. Screenshot –


Now if you do a “#lsscsi” this will display four (04) disks. Previously it was three (03) disks in this example.

The new disk information will appear in “dmesg” as well; do a “dmesg | grep disk” to find details.

Next step should be partition the new disk, create file system and provide a mount point; if you want auto mount then add the partition details to “/etc/fstab”.