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Thin disks grow as data is written; traditional thick disks reserve their full configured capacity when created. That difference can make a datastore look full even when guests have not filled their virtual disks. To find what is actually using space, separate the disk’s configured capacity from its datastore consumption—and, where applicable, from array-side usage or vSAN policy reservations.
What “eating the datastore” can mean
Capacity figures describe different things. A virtual disk’s provisioned size is its configured maximum; its used size is the storage currently consumed at a particular layer. A thin disk can have a large provisioned size while consuming less space on VMFS. A traditional thick disk reserves its full configured size at creation, so datastore usage can rise to that amount before the guest fills the disk. Array-side thin provisioning and vSAN policies can further change what a reported “used” or “reserved” figure represents.
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These definitions describe VMware vSphere virtual disks on VMFS. Check the behavior for your vSphere release, datastore type, storage policy, and storage array before treating a displayed number as physical consumption.
How thin and thick virtual disks allocate space
| Disk type | Capacity at creation | When blocks are zeroed | Practical trade-off |
|---|---|---|---|
| Thin | Does not allocate the full disk capacity up front | Blocks are prepared as they are first consumed | Can save space when guests use only part of their configured disks, but requires monitoring because provisioned capacity can exceed backing capacity. |
| Thick lazy-zeroed | Allocates the full disk capacity | Zeroing is deferred until first use of the relevant blocks | Reserves datastore capacity in advance, but first use can still involve preparation work. |
| Eager-zeroed thick | Allocates the full disk capacity | Blocks are zeroed at creation | Does preparation up front, using capacity and creation time before the guest writes data. |
Broadcom distinguishes these three formats in its vSphere provisioning documentation. “Thick” is not a single behavior: lazy-zeroed and eager-zeroed thick disks allocate capacity at creation, but differ in when zeroing occurs.
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Why thin provisioning can run out of space
Thin provisioning makes efficient use of capacity when virtual disks are configured larger than their guests’ actual needs. The trade-off is that multiple disks can collectively promise more space than the datastore—or the storage pool beneath it—can physically provide. If those disks grow, the backing storage can run out. With thin provisioning at both the virtualization and array layers, a reassuring figure at one layer may not show how close the underlying pool is to exhaustion.
Track available capacity and growth at both the datastore and storage-pool layers. Include snapshots, swap, and other VM files in the accounting; virtual-disk allocation alone may not explain the total. Microsoft’s thin-provisioning guidance describes alerting as capacity approaches exhaustion and status mechanisms for resource allocation. The right alert point depends on the product and configuration, so there is no universal free-space threshold to apply to every deployment.
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Does thin provisioning make a VM slower?
There is no universal performance winner. Broadcom says performance is similar across provisioning types during normal production, while large-scale sequential writes can expose differences as new blocks are consumed. Thin disks have not allocated or zeroed those blocks in advance; lazy-zeroed thick disks still defer zeroing; eager-zeroed thick disks have completed the preparation at creation. As a result, new-block preparation may add work during large writes on thin or lazy-zeroed disks.
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How to diagnose datastore usage
- Compare provisioned with datastore-used capacity. Look at each virtual disk’s configured size and the storage it currently consumes; do not treat those values as interchangeable.
- Identify the disk format and storage context. Determine whether each disk is thin, thick lazy-zeroed, or eager-zeroed thick. Check whether vSAN policy reservations or array-side thin provisioning affect the figures you see.
- Account for other files. Check snapshots, swap, and other VM files before attributing all datastore consumption to virtual disks.
- Review growth and backing capacity. Check available space and trends at both the datastore and storage-pool layers, paying particular attention to thin-on-thin configurations.
VMware’s guidance on vSAN thin provisioning and thin-on-thin deployments provides context for why policy reservations and underlying array behavior should not be confused with traditional VMFS thick allocation. The posts date from 2020 and 2022, so confirm details against your current vSAN version and policy.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When conversion makes sense
Changing a disk’s provisioning type can be an option when capacity planning or a specific workload calls for it, but conversion is an operational change—not a substitute for understanding the storage layers. Broadcom advises making a valid backup and confirming sufficient capacity before conversion. Its conversion guidance notes that the time required depends on array performance. Plan for the destination’s space requirements and the workload impact rather than assuming the operation will be immediate.
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