Database Reference
In-Depth Information
use flash SSDs, EFDs, and PCIe that can greatly improve SQL performance, directly in
the VMware ESXi servers hosting SQL. This is where server-side flash and associated
acceleration solutions come in. Server-side flash when used as part of an IO
acceleration solution can be thought of as cheap memory, rather than expensive disk. It
is definitely cents per IOP and dollars per GB, but the returns on investment and
performance can be substantial. Especially when it is not possible to add more RAM to
the buffer cache, which would be the fastest possible storage from a performance
perspective.
By using server-side flash acceleration, you can normally consolidate more SQL VMs
per ESXi host, with less memory directly assigned to each SQL VM, and without
sacrificing performance and user response times. Read or write IOs are offloaded to the
local server flash device, and this acts as a very large cache. It can also greatly reduce
the load on the back-end storage, which allows the array to improve its efficiency.
Because the flash devices are local to the server, the latencies can be microseconds (us)
instead of milliseconds (ms) and eliminate some traffic that would normally have gone
over the storage network. By reducing the storage IO latencies, not only are user
response times improved, but overall server utilization is improved. You may see
increased CPU utilization, as you are able to get more useful work done by reducing
system bottlenecks.
In this section, we cover three different server-side flash acceleration solutions that are
supported with VMware vSphere and can greatly improve the performance of your SQL
databases. The solutions we cover are VMware vSphere Flash Read Cache (vFRC),
which is included with vSphere 5.5, Fusion-io ioTurbine (IOT), and PernixData Flash
Virtualization Platform (FVP). The first two solutions act as a read cache only, as all
writes go directly to the backend storage while being cached and are therefore write
through. PernixData FVP, on the other hand, offers a full write back cache, where both
read IO and write IO can be accelerated.
VMware vSphere Flash Read Cache (vFRC)
vSphere 5.5 introduces vSphere Flash Read Cache, or vFRC, which is an infrastructure
layer that aggregates flash devices into a unified flash resource pool. vFRC supports
locally connected flash devices such as SAS/SATA SSDs and PCIe. The flash resource
can be used to cache read IOs and is configured on a per-VMDK basis. The vFRC write
policy is write through, which means that all writes go to persistent storage and are
cached in vFRC simultaneously. To prevent pollution of the cache, large sequential
writes are filtered out. Each VMDK flash resource allocation can be tuned based on the
workload. For SQL, it's recommended that data file VMDKs and Temp DB VMDKs be
configured for vFRC when used, whereas transaction log will usually have little benefit.
Figure 6.39
shows a high-level overview of the VMware vSphere Flash Read Cache
