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rdnair
Enthusiast
Enthusiast

Sizing LUN for enough free space

Hello All,

As a good practice how much free space should I leave on a LUN?

Let me be a little more specific. I need a separate dedicated LUN for a VMDK (D drive on a Windows server) of around 250 GB. This LUN holds no other file/data, but just the VMDK file for the D partition of a Windows server. I will be using VRanger to backup the VM, so snapshots will/should be on the LUN where the config file and primary vmdk are stored, which has enough free disk space (100 GB).

In the above scenario how much free disk space should I leave on the LUN to host a 250 GB VMDK?

Thanks in advance.

Rupesh

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Troy_Clavell
Immortal
Immortal

taking http://kb.vmware.com/kb/1012384 into consideration, I would go with 300GB LUN with 2MB block size

rdnair
Enthusiast
Enthusiast

Troy,

Thanks for the suggestion. Considering I opt for a 250 GB thick provisioned disk, and this LUN does not keep any snapshots, don't you think 50 GB is a lot of overhead?

Rupesh

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a_p_
Leadership
Leadership

Actually you will not need much more space as the size of the VM. You should leave some space for management (VMFS).

However, what I'm not sure at the moment is, where VMware places the ctk file (Changed Block Tracking) which can be used to optimize backup.

So, 5 GB additional disk space should be fine.

André

rdnair
Enthusiast
Enthusiast

Thanks a.p. I assume the CBT files are kept in the LUN where the vmx (config) files are. In my example it would the LUN where I have the primary VMDK and the vmx (config) file. Please correct me if I am wrong.

Regards,

Rupesh

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golddiggie
Champion
Champion

Never exceed 90% utilization on any LUN... So, if you're only going to put 250GB on the 300GB LUN, you should be fine.

You will get alarms going off if you do exceed the 90% threashold, so you're better off planning to avoid that from the start...

VMware VCP4

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a_p_
Leadership
Leadership

Yes, I was not sure but found it on What is Changed Block Tracking in vSphere?

You might wonder where CBT stores the information about changed blocks for a virtual disk, it does this in a special “-ctk.vmdk” file that is created in each VM’s home directory for each virtual disk that it is enabled on.

André

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rdnair
Enthusiast
Enthusiast

golddiggie,

Thx for the response. You mean the threshold should not exceed 90% because of the alarms? or is there any other reason?

I am not worried about the alarms. I can disable the alarm features for any LUN if needed.

Regards,

Rupesh

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golddiggie
Champion
Champion

The alarms are one reason, but I also believe you'll have a performance hit once you start going over that level (or within that range)... It's part of the BKM's for VMware environments...

VMware VCP4

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mcowger
Immortal
Immortal

There isn't anything magic that happens at 90%. We regularly run LUNs to 98% and 2GB free with no performance problems (but we can only do that because you dont use snapshots or anything).

If you are cool to ignore the alarms (we disable them and do our own separate alarming), go for it.

--Matt

VCP, VCDX #52, Unix Geek, Storage Nerd

9773_9773.gif

--Matt VCDX #52 blog.cowger.us
rdnair
Enthusiast
Enthusiast

Thanks Matt. That means in my example above, for a 250 GB thick provisioned disk I could create a LUN 260 GB and leave around 5-10 GB free disk space without any problems.Right?

Regards,

Rupesh

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mcowger
Immortal
Immortal

Assuming no snapshots, yes.

Not sure off the top of my head if the snapshot file for the 250GB 😧 drive will go on the datastore with the VMDK or the one with the VMX.

--Matt

VCP, VCDX #52, Unix Geek, Storage Nerd

9773_9773.gif

--Matt VCDX #52 blog.cowger.us
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a_p_
Leadership
Leadership

Snapshot files are always created in the VM's base directory, unless you explicitly redirect them.

see http://kb.vmware.com/kb/1002929

André

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