Types of virtual disks, Question?

Hi all,
I am newbie in vmware and I want understand it as deep, so I have question about :
"Thick Provision Eager Zeroed – the space required for the virtual disk is allocated during the disk creation. The space is zeroed out when the disk is created.

Thick Provision Lazy Zeroed – this is the default disk type. Space required for the virtual disk is allocated during creation. The space is zeroed on command on the first write from the virtual machine. "


 Can you explain  simply  zeroed out and what do it?

Thanks,
Hamid SaeedAsked:
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Andrew Hancock (VMware vExpert / EE Fellow)VMware and Virtualization ConsultantCommented:
Basically at the time the virtual disk is created every single sector has a zero written to it. E.g. inside the VMDK so it's a clean disk and has no used data from the datastore.

This can take time to create the disk but zeroed out disks can increase performance.
gheistCommented:
You talking vmfs or nfs?
Hamid SaeedAuthor Commented:
Hi,
Yes, my question is about create virtual diskd for a VM,p in when I want create a VM with wizard ,
Ok,but still, I don't understand why "zero written" operation must be do?
Can you compare it with  prepare a physical  disk ?

Thanks dearm
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gheistCommented:
Because all no-windows nfs servers and vmfs allocates sparse files in lazy case, sort of thin provisioning in storage unit
Andrew Hancock (VMware vExpert / EE Fellow)VMware and Virtualization ConsultantCommented:
It clears the physical sector of the disk

Excellent.g. zeros overwrites blanks the disk destroys the data in the sector before use.

Eager zeroed does this to the entire disk at creation.

Lazy zeroed does it as the disk is written to by the OS so there is a slight delay in writes...
gheistCommented:
Actually you can read lazy zeroed disk and it is all zeroes in untouched blocks. What happens behind the scenes is that your storage (VAAI) driver makes never written sectors up as zeroes.
Hamid SaeedAuthor Commented:
Hi and very thanks,
So, for writing to virtual disks must first write zero and data overwrite in place?
Is it right?
And do it is right for physical disks,too?

Really thanks for you,
Andrew Hancock (VMware vExpert / EE Fellow)VMware and Virtualization ConsultantCommented:
Correct only for virtual disks.

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