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What are some of the downsides of VMware thin provisioning?
There are two main disadvantages to VMware's vStorage thin provisioning. The first is that thin provisioning makes it likely that physical storage will be overprovisioned. This isn't necessarily a big deal in and of itself, but the potential exists for overprovisioned storage to run out of space if storage capacity isn't carefully monitored. However, VMware includes some mechanisms that reduce the chances of this happening.
The second disadvantage is that thinly provisioned storage tends to not perform as well as storage that has been thick-provisioned for the following reasons:
- Storage fragmentation. Thinly provisioned storage only allocates blocks on an as-needed basis, so it can be susceptible to a higher degree of fragmentation than storage that has been thick-provisioned. Keep in mind that this is not an absolute. Whether or not fragmentation becomes an issue depends heavily on how the storage is used and on the capabilities of the underlying storage hardware. For instance, some storage hardware uses a write algorithm designed to minimize fragmentation. Likewise, fragmentation isn't an issue for solid-state storage.
- VMDK thin provisioning. If a VMDK has been thinly provisioned, the VMFS data store has to be locked every time a new block is added to the VMDK so the metadata can be updated. This can lead to performance issues if there are many VMDKs growing at the same time.
In all fairness, this locking process was originally done with SCSI reservations. Today, alternative methods are used based around VMware's vStorage APIs for Array Integration and Atomic Test and Set. These newer methods are less susceptible to performance problems than the SCSI reservation method.
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