What challenges can arise from implementing automatic thin provisioning on VMware storage?
When using automatic thin provisioning, users can create virtual disks (VMDK files) that appear larger than the actual available physical storage space. This feature allows the guest operating system to operate under the assumption that ample storage is available, even when physical capacity is limited. However, this approach can lead to several potential challenges.
Firstly, one of the most significant risks is running out of physical storage space. Since the guest OS cannot detect when the physical storage is full, this can lead to unforeseen space shortages. To address this issue, starting with vSphere 5.0, VMware introduced a warning mechanism that triggers when storage usage reaches 75%, and it stops Storage DRS (Distributed Resource Scheduler) from placing new workloads on thinly provisioned datastores that are nearing capacity, thus preventing over-provisioning.
Secondly, VMware has improved how virtual machines handle situations where storage space is insufficient. Previously, if physical storage was exhausted, affected VMs might be forced to shut down. From vSphere 5.0 onwards, as long as the VMs do not require additional storage space, they can continue to function normally.
Additionally, another challenge associated with thin provisioning relates to the reclamation of storage space. When data is written to VMDK files on a thinly provisioned datastore, it consumes physical storage space; however, when this data is deleted, the corresponding physical space is often not immediately reclaimed. Prior to vSphere 5.0, this was especially problematic, particularly in scenarios where VMs were deleted or migrated. Although vSphere 5.0 introduced a mechanism to notify the storage array about which blocks have become idle, early implementations of the UNMAP functionality had performance issues and were at one point recommended to be disabled. By vSphere 5.5, VMware released the ESXCLI command to manually clear out blocks that have been reclaimed, improving this process.
Therefore, when employing thin provisioning, administrators need to closely monitor storage utilization and health to mitigate the various challenges mentioned above.