Think or Thick Disk

http://www.viveknadar.com/inquisitive/2009/06/vmware-tip-4-thin-provisioning-with-vsphere.html

VMware Tip 4: Thin provisioning with vSphere - Tips & Tricks


With vSphere 4, thin provisioning was supported at virtual disk level through vCenter GUI. With thin provisioning, networked storage space utilization is improved because of dynamic allocation of storage to the virtual machines. vSphere has built in capabilities to provision, manage and monitor thin provision storage to virtual machines



Thin Provisioning Tips & Tricks :
•Monitoring the datastore space utilization is very critical to prevent running out of space by overcommitment. Leverage vSphere Datastore alarms like "Datastore usage on disks" and "Datastore disk overallocation %" to trigger alerts

•Take virtual disk size, snapshots, swap and other files into consideration while planning for storage space for datastores and not just virtual disk size

•Avoid running guest OS disk defragmentation programs on a virtual disk which is thin provisioned as it might inflate the disk. Using storage vMotion using thin option will defragment the virtual disk

•Use storage vMotion to convert a existing thick virtual disk to thin

•Use thin/virtual provisioning at storage array also if it supports for better efficiency

•Enable alerts on the storage array to monitor the utilization of thin filesystem/LUN

•As deleting data inside the virtual disks cannot claim the space back, storage vMotion using thin option can be used to make the disk thin again.

•VMware FT cannot be used with thin provisioned virtual disks. Enabling FT will make the virtual disks thick

•Avoid using thin virtual disks for applications in guest os which zeroes out the space before actually writing into it



Different virtual disk formats :


Thin - vmkernel allocates space only before an I/O is committed. Virtual disk size will be equal to the utilized size and will grow till the user specified size


Zeroed Thick - vmkernel zeroes out the required free space only before an I/O occurs. Virtual disk size will be equal to the user specified size during virtual disk creation



EagerZeroed Thick - vmkernel zeroes all the free space during the creation of virtual disk. Virtual disk size will be equal to the user specified size during virtual disk creation


VMs having virtual disk with thick formats can be converted to thin format using storage vMotion in vSphere


Great Thin provisioning resources :



http://virtualgeek.typepad.com/virtual_geek/2009/04/thin-on-thin-where-should-you-do-thin-provisioning-vsphere-40-or-array-level.html



http://itknowledgeexchange.techtarget.com/virtualization-pro/pointers-for-using-thin-provisioned-disks/


http://www.vmware.com/files/pdf/thin_provisioning_datasheet.pdf