Information Technology Reference
In-Depth Information
In such cases, SIOC can detect “external work loads” and w ill automatically stop throttling. However,
at the next latency evaluation period (4 seconds), SIOC will again check the latency of the datastore
against the congestion threshold and see if it needs to start throttling again, and the cycle starts
again.
To resolve this issue, VMware recommends that you avoid sharing physical disks across both vir-
tual and nonvirtual workloads. Because of the architecture of some arrays, this may be di cult,
so check with your storage vendor for their recommendations and best practices.
So far we've discussed shared storage where the features and settings are all array agnos-
tic—Fibre Channel, iSCSI, or NFS—it really doesn't matter how it's presented to your ESXi hosts.
In the next section, we'll introduce some features that are solely for use with local SSD-based
storage.
Utilizing Flash Storage
As physical to virtual ratios increase and environments become denser, the need for faster
storage is obvious. Flash storage is quickly becoming an industry standard, and it's not uncom-
mon for both servers and SANs to ship with l ash-based storage. Traditionally, SANs have used
smaller portions of l ash storage as caches to increase the response times to slower spindles, but
as SAN sizes increase, so does the need for expensive l ash cache. Depending on the scenario, a
more cost-effective solution can be to load up servers (either rack mount or blade) with local SSD
or PCIe-based l ash storage.
vSphere 5.5 offers two completely different ways of using local l ash-based host storage,
whether it be SSD drives or PCIe cards. vFlash Cache (vFC) is a feature that acts as a buffer for
I/O on a per-VM basis. The other feature, Swap to Host Cache, is used to allocate local l ash
disks as swap space. These features are not mutually exclusive; they can be enabled at the same
time and can even be backed by the same SSD, but they work in completely different ways.
Flash or SSD?
Although the industry may use diff erent names, all fl ash-based storage is built on similar NAND
technology. SSD, EFD, or just fl ash is just nonvolatile memory that retains its data even without
power. h e diff erentiator between the diff erent products is similar to “regular” hard disks—latency,
bandwidth, resiliency, and of course, size. And just as with regular spinning disk drives, there can
be very diff erent performance and capacity characteristics depending on the model.
Enabling vFlash Cache for Virtual Machines
vFlash caching is a resource type that can be allocated on a per-VM basis just like CPU or
memory. In fact, it can be allocated down to an individual virtual disk (VMDK) level. However,
a VM does not
need
to have vFlash allocated to function. Just like CPU and Memory allocations,
