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Just like Windows..VSphere also creates dedicated Flash Read Cache

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Microsoft isn’t the only hypervisor vendor to use flash-based caching. VMware allows high-speed caching through its vSphere Flash Read Cache feature.

VMware’s approach has a couple of things in common with that of Microsoft’s. VSphere Flash Read Cache is intended to reduce latency by making strategic use of flash storage. The caching process is also completely transparent, and VMs are oblivious to the cache’s existence. Therefore, no cache-related agents are required.

This is where the similarities end. Microsoft’s approach involves building virtual hard disks on storage tiers controlled by the Windows Server OS. In contrast, VMware treats flash storage as a provisionable resource. VSphere allows for the creation of CPU pools and memory pools. Flash-based caching is based on the creation of a logical object called the Virtual Flash Resource.

The Virtual Flash Resource is nothing more than a logical grouping of flash storage capacity (essentially, a pool of SSDs). Even so, there are some important things to know about it:

  • Flash storage must be dedicated to the cache. SSDs can’t be shared by a SAN or NAS and the Virtual Flash Resource — they must belong to one or the other. Similarly, you can’t place a VMware data store on Virtual Flash Read Cache storage.
  • It’s a host-level object. In other words, the cache is used by resources on a specific host server. The cache isn’t a cluster-level object, and the contents aren’t replicated among cluster nodes. However, vMotion is flash-cache-aware. Administrators can choose to include cache contents when using vMotion to move a VM to another host, or the cache contents can be abandoned. If the cache contents are copied, the destination host must have its own Virtual Flash Read Cache.
  • There’s a penalty for using the Virtual Flash Read Cache in conjunction with vMotion. If the cache contents are included in the vMotion operation, it will take longer to move the VM than it would have if the cache did not exist or if the cache contents hadn’t been migrated. Unfortunately, it’s difficult to estimate the amount of extra time the cache contents add to the vMotion process since the duration is based on such variables as cache size and the amount of available network/storage bandwidth.
  • There’s a penalty for not including the cache contents in vMotion operations. If the cache contents aren’t included in a move, the vMotion process will take the same amount of time to complete as it would if caching wasn’t used. The performance penalty comes into play after the vMotion completes because the VM will no longer have storage blocks stored on an SSD cache. The performance of the VM will eventually recover, but the cache will have to be rebuilt first. This process is similar to when a flash-based cache is first added to a VM and vSphere has to learn which data should be cached.

In case you’re wondering, most Hyper-V deployments are based on the use of shared storage. If all the Hyper-V hosts use the same physical tiered storage, a live migration shouldn’t impact the contents of the cache.

Although the VMware vSphere Flash Read Cache is designed to boost VM performance, the VMs don’t access the cache directly. Instead, a component known as the vSphere Flash Read Cache Infrastructure acts as a broker that controls flash cache usage. The vSphere Flash Read Cache Infrastructure also enforces administrative policies related to the cache.

While the primary job of the vSphere Flash Read Cache Infrastructure is to broker VM cache access, it also allows the hypervisor to utilize the cache through the Virtual Flash Host Swap Cache feature, which replaces vSphere 5.0’s Swap to SSD.

Whether you’re working in a Microsoft or a VMware environment, flash-based caching has the potential to greatly improve VM performance. The key to receiving the greatest benefit from this caching is to understand how your hypervisor uses the cache, then to add flash storage in a way that adheres with the established best practices for your hypervisor.

Source: Associated press

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This entry was posted on December 19, 2014 by in VMware and tagged , .
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