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Use flash with vSAN, ESXi virtual flash caching and VMFS data stores
ESXi can use flash storage in a variety of ways. Ultimately, however, the degree to which ESXi supports flash storage varies depending on the hypervisor version.
Like other hypervisors, VMware ESXi fully supports flash storage. There are three distinct ways ESXi supports the...
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use of flash storage: using vSAN, ESXi virtual flash caching and Virtual Machine File System data stores.
The first way ESXi supports solid-state drive (SSD) storage is with vSAN. VSAN doesn't just support SSDs, it requires it. According to the hardware requirements for vSAN, the cache requires a Serial Advanced Technology Attachment (SATA), a Serial-Attached SCSI (SAS) SSD or a PCIe flash drive. This flash storage device must be dedicated to the vSAN cache; it can't be shared with the vSphere Flash Read Cache, and it must not be formatted with a file system.
VMware vSAN also requires that flash storage be available for VMs to use. VSAN supports two different configurations: a hybrid group configuration and an all-flash disk group configuration. Needless to say, an all-flash disk group configuration relies solely on SATA, SAS SSDs or PCIe flash devices.
Hybrid group configurations are made up of hard disk drives (HDDs), but are considered hybrid because of their reliance on the flash-based caching tier.
ESXi virtual flash caching
A second way that ESXi supports flash storage is in the form of virtual flash resources. For example, you can use flash media to create a virtual Flash Read Cache for VMs. This cache improves the performance of individual VMs. You can configure a virtual Flash Read Cache for any VM. Each cache corresponds to an individual virtual hard disk, so a VM with two virtual disks could have two ESXi virtual flash caches.
Flash storage vs. hard drive storage explained
VMware attempts to conserve flash resources by dynamically creating the flash cache when a VM is powered on -- assuming that the VM is configured to use a cache -- and then automatically removing the cache when the VM is shut down.
Another option is to use flash storage to set up a host swap cache. This method uses flash storage as a write-back cache for VM swap files. Unlike an ESXi virtual Flash Read Cache, this method shares the cache among all of the VMs running on the host.
VMFS data stores
A third way ESXi supports flash storage is by enabling Virtual Machine File System (VMFS) data stores to be stored on flash media. It isn't surprising that VMware enables data stores to reside on flash storage; they can reside on any media that VMware officially supports. But VMware makes a data store's underlying flash storage transparent to the guest OSes of the VMs within the data store. In other words, ESXi makes it so a VM's OS can tell that it is running on flash storage.
This capability is significant because some OSes optimize storage I/O based on the type of media that is being used. For example, if an OS runs on HDD storage, then it might periodically perform a disk defragmentation in an effort to improve storage performance.
If that same OS runs on flash storage, then a disk defragmentation would be counterproductive. Not only would defragmenting an SSD do almost nothing to improve performance, but the operation would likely shorten the disk's life span.
For a guest OS to detect the use of flash within a data store, the data store must use VMFS5 or later. Similarly, the host must run ESXi 5.x or higher, and the VM must use virtual hardware version 8 or higher.
Finally, if a data store is configured to use extents, then all of the physical extents must use flash storage. Otherwise, virtual disks residing on the data store won't be identified as ESXi virtual flash.