Understanding VMware Capacity - Measuring Memory Capacity (6 of 10)

Today we'll take a look at measuring memory capacity, below is a typical graph we might use to understand how memory is being used by a VM. 

The VM has been granted about 4GB RAM. That’s what the OS thinks it has to use.

The host memory in use, shows how much physical RAM is currently being used by the VM. We can see that at 09:00 this increases and at the same time shared memory reduces.

Memory Used is our term for Active memory, and remains steady throughout, as does the Memory Overhead on the host used to support this VM. What we can see is that only about 400MB of memory is being accessed on a regular basis. Between 1.5GB and 2GB of memory is not unique to this VM and is shared with others.

Balloon Driver

The balloon driver takes about 2.5GB of space in the memory of the VM. This will cause the OS to swap out to disk until it cannot do any more, and then the hypervisor starts to swap out memory also. At that point, performance is likely to be impacted.

Here's an example of the balloon driver in use.

Where ballooning is persistent (rather than an occasional spike), then changes should be made in order to ensure there is enough RAM available for the VMs in the cluster. Ballooning itself has an overhead on the hypervisor and as such there is the potential to impact performance for the host. Changes don’t necessarily have to be more RAM installed. The very first thing to consider is if any VMs have been created and “forgotten” about. On Monday I'll take a look at Cluster memory.

Phil Bell

Consultant

Understanding VMware Capacity - Measuring Memory Capacity (5 of 10)

Memory capacity is generally the limiting factor in a VMware cluster. This is improving as hardware specifically designed to be a host has become available with plenty of RAM able  to be installed, but clusters still generally have less Memory headroom than CPU. Memory on VMware is not just a case of monitoring % used. Again like CPU we want to be able to compare memory utilizations in comparable metrics, so that’s MB or GB. Then we move on to the specific VMware type metrics.

Reservations

A VM or a resource pool can have a reservation. Which means if it needs that memory, it gets to use it - no questions asked. However you cannot reserve more memory than exists in the host server.

Limits. Just because a VM has been configured to have 8GB RAM, doesn’t mean you can’t set a lower limit of say 4GB RAM. At which point, if the VM tries to use more than the limit, then some data will be placed into the Swap file of the OS or VMware may balloon some of the VMs memory to free up pages that the OS can use.

Ballooning

When VMware wants to allocate memory to a VM, but there is a shortage of Memory, then a balloon may be inflated inside the memory of one (or more) VMs. This balloon pins itself into RAM and cannot be swapped out, thus forcing the OS to swap some memory out to disk. The pages pinned into memory are not all actually stored in memory on the Host, as it knows the contents of the balloon are not important. This then frees up pages, that the Host can allocate the VM it wanted to give memory too. One of the reasons this happens is that an OS will use a page to store some data, then later the program will no longer need that data, and the OS puts that page on the free list. However, the hypervisor has no idea the page is no longer needed. The data in the page has remained unchanged. In order to identify free pages when the balloon expands and the OS put it into the free pages, the hypervisor can

identify the balloon memory pages, and therefore the pages that the OS is not using for other processes.

Shared pages

Shared Pages are pages in memory that are identical. Rather than store duplicates of the same page, the hypervisor will store a single copy, and point the appropriate VMs to it. This works well where servers are running the same OS and doing the same job, and therefore much of the memory in use is identical.

Active Memory

Active memory is the amount of memory that has been accessed in the last 20 seconds. Having sufficient memory to contain the active memory of the VMs is crucial to performance.

Memory Available for VMs

Memory available to the VMs shows what it says. There is some memory in the cluster that VMs cannot use as it’s being used by the Hypervisor to support the VMs. The memory left over is available.

On Friday I'll talk about understanding how memory is being used by a VM. In the meantime don't forget to sign up for my next webinar 'Top 5 VMware Tips for Performance and Capacity'

https://www.metron-athene.com/services/webinars/capacity-management-webinars.html

Phil Bell

Consultant

Virtualization Oversubscription - What’s so scary? Memory stats(10 of 20)

Let’s look at some stats for a single Virtual Machine

This is a 4GB VM, but it’s only accessing about 400 MB on a regular basis.  It’s got 2.6GB of memory that’s unique to itself, and 1.4GB that’s shared with other VMs.

So at least one other VM is likely to be sharing at about 1.4GB memory as well.  Given there are a lot of windows VMs in that cluster it’s likely a lot of them have similar amounts of shared memory.  If there are 10 VMs on that host then that’s about 15GB or RAM that you don’t have to have installed or rather, a few more VMs that will fit on the host.

There are also a couple of hours where the balloon driver steals some memory from the VM.  Only about 50MB and given the VMs only accessing 4 to 500MB of RAM, out of the 2.6GB that it’s using, the OS probably just released some cache to satisfy that request.

On Friday we'll be looking at Reservations and how they work.

Don't forget that registration is open for our next webinar 'Hardware's a commodity - Why bother managing capacity? http://www.metron-athene.com/services/webinars/capacity-management-webinars.html

Phil Bell

Consultant

Virtualization Oversubscription - What’s so scary? VMkernel Swap (9 of 20)

A reservation is typically set against a resource pool and filters down to give a VM rights against memory.  Essentially if a reservation has been set and applies to this VM, then the VM is guaranteed that amount of memory will be made available in RAM on the EX host.  You can never reserve more memory than exists.  So reservations can ensure good performance for the VMs you care about.  You put the VMs in a resource pool, and allocate a reservation that’s appropriate.  That might be your 1:1 ratio with allocated and reservation.  Then let other less important VMs worry about oversubscription.

When an ESX host is very short of memory it may have to resort to using .vswp swap files for the VM memory.  At this point performance will be affected as data that the OS believes is in memory is, in reality, now on disk.

A VM as default can have up to 65% of its memory used by the balloon driver.  It may also have a memory reservation.  The reservation cannot be swapped or taken up by the balloon driver.  Any memory outside the 65% used by the balloon driver, and the reservation, can be placed into a .vswp file.

In reality you never want this to happen.

On Wednesday I'll be discussing Memory.

Phil Bell

Consultant

Virtualization Oversubscription - What’s so scary? Memory test (8 of 20)

If memory must be copied to or from disk because there is more requested than can be satisfied, what’s the penalty for doing this?

•        Memory vs. disk speed is…?

A) Memory is 100x faster than disk

B) Memory is 1,000x faster than disk

C) Memory is 10,000x faster than disk

D) Memory is 100,000x faster than disk

E) Memory is 1,000,000x faster than disk

F) I have no memory of the event, your honour

A modern disk will respond to an I/O in about 5 milliseconds (5 * 1/1000 of a second).  Access to memory is usually in the order of 50 nanoseconds (50 *1/1,000,000,000 of a second).

That makes disk access a hundred thousand (100,000) times SLOWER than memory access. Tiny numbers like this are difficult to comprehend, so imagine that the memory access time was 1 second.  To write something to disk would then take about 27 ¾ hours to complete.

That’s one good reason for avoiding swapping if at all possible! On Monday I'll be looking at vKernel Swap.

In the meantime why not take a look at some of our VMware on-demand webinars and white papers. Join our Community via our Resources section and get free access.

http://www.metron-athene.com/resources/login.asp

Phil Bell

Consultant