Tuesday, 17 January 2017

Understanding, Zeroing, Lazy Zeroed, Eager Zeroed, and Thin Provision concepts in VMware vSphere.

Or, VMware Disk Provisioning Concepts about Lazy Zeroed, Eager Zeroed and Thin Provision.
Or, Lazy Zero Vs Eager Zero Disk Provisioning.
Or, Difference between Thick Provision Lazy Zeroed, Thick Provision Eager Zeroed, and Thin Provision Disk Provisioning.

Before the VM can actually use the allocated drive space, there are some operations that take place on ESXi host level. When you allocate a disk space from your Datastore, Datastore also maintain itself to allocate that amount of Disk space with clean/formatted sectors.

This disk sectors cleaning process is basically referred as Zeroing term. As you do disk formatting at Guest level to make it ready to use, the same task is performed by ESXi Host at Datastore level when you add any partition/disk to a VM from any Datastore.

Thin Provisioned Disk: Allocate and zeroed on first write. When you create a disk with thin provisioning method, it only writes a small amount of metadata to the Datastore where it resides. When the write operation is performed, vSphere first updates the allocation metadata of the VMDK, then zeros out the blocks, lastly it writes the data. Thin provisioned VMDKs have the lowest performance logically at some points.

Once the Zeroing is completed for allocated disk blocks, almost all disk types performs equally.

Thick Provisioned Lazy Zeroed Disk: Allocate in advance and zero on first write. Do nothing to empty blocks, it zeros all the blocks at first access... Provisioning of this disk type is little faster than other disk type. Performance of this disk type is slower than Eager Zeroed Disk type and it is not supported for clustering.

Example: If you have 10GB disk size and having only 5GB data in it, then 5GB will be zeroed at first access, rest 5GB will be left.

Thick Provisioned Eager Zeroed Disk: Allocate and zero in advance. Writes 0 to empty blocks. Write is performed on whole allocated disk.  Means, its pre-zeroing all the blocks on the disk. Performance of this disk type is better than all others but provisioning time may take little longer than other disk types. 
It also supports clustering features.

Example: If you have 10GB disk size and have only 5GB Data in it, then 5GB will be used and rest 5GB will be written with 0. No further Zeroing is required while any write operations.

Cheers, Please write me back if you have any query or feedback on this..

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