As virtualization has grown into a standard in almost all data centers, so too has the use of shared storage. Today, there is very little use for traditional local storage on a vSphere host. In fact, given the small storage footprint of ESXi, most server manufacturers now offer a local SD card slot so admins can install and run ESXi from an SD card to reduce the power, heat and cost of local hard drives. But, the declining cost of solid-state storage and recent VMware SSD updates have caused more organizations to consider using solid-state storage on their hosts. Here are two of the more compelling use cases for local solid-state storage on an ESXi host server.
VMware SSD support: Swap to local cache
A new feature in vSphere 5 allows a host to automatically detect a local solid-state drive (SSD) and allow for part of that disk to be set up as a local cache for VM swap files. In some cases, this VMware SSD configuration can greatly improve performance.
When a host starts to run out of RAM capacity, it takes several steps to free up RAM to meet the needs of the VMs. The host starts by eliminating redundant pages in RAM using Transparent Page Sharing. Next, a host turns to ballooning to force in-guest memory pressure, and then memory compression to reduce the size of pages in RAM. If the host still cannot free up enough RAM, then it will be forced to swap out memory pages to disk. The Swap to Host Cache option, if enabled at the host configuration section of the vSphere client, will allow the swapping to first occur to the local SSD before being swapped to the shared disk. This VMware SSD support results in a significantly reduced performance hit to a VM when compared with swapping to a shared disk, because of the lower latency and faster performance of a local SSD.
VMware View linked clones
In a VMware View environment, disk I/O is often a performance bottleneck. This can lead to expensive shared storage solutions to provide the proper IOPS for the virtual desktops. Utilizing local SSDs is one way to reduce the cost of the shared storage solution. Admins can implement this in one of two ways, both of which require all hosts to have local SSD storage.
- Place the golden image (replica) on shared storage and the linked clones on the local SSDs. This approach will help reduce the shared disk space needed.
- Place both replicas and linked clones on the local SSD. This approach increases the amount of SSD space necessary on each host, but drastically reduces the performance requirements of the shared storage array. The reduced cost of the shared storage array can often justify the extra cost of the local SSD space.
In both cases, since the VM is stored on local storage, the virtual desktops cannot be live migrated. Using this approach will require more planning when performing host maintenance since the desktops will have to be powered down in order to reboot host servers.
VMware SSD alternatives
Most server platforms now offer SSDs that fit the form factor of SAS or SATA drives, making it easy to add local SSDs. Alternative solid-state storage options are now appearing on the market offering even higher performance benefits by eliminating the SAS or SATA bus and plugging directly into the PCI bus. VMware currently supports offerings from Fusion-io and LSI.
EMC recently introduced a new product, called VFCache, a PCI solid-state storage device used to extend the SSD-based cache capabilities of their arrays into the host servers. This provides better I/O performance to commonly read blocks of data by caching them locally on the host server.
Although most of these solutions are currently solving niche pain points, the industry seems poised for a renaissance of fast local storage to increase disk performance while also reducing the performance needs of the shared storage arrays.