Database Reference
In-Depth Information
ESXi host experiences an outage, it is vSphere HA's job to restart the affected virtual
machines on another ESXi host in the cluster. This provides a recovery time measured
in the reboot of a virtual machine. vSphere HA is turned on by a check box for an entire
vSphere cluster: no complex configuration or special skill sets required. The value this
brings for SQL Server virtual machines is twofold.
First, by virtualizing standalone SQL Servers, this automatically provides them
hardware-level protection. In the physical world, if there is a hardware failure, there is
a service interruption until the hardware issue is resolved. This usually translates to
downtime measured in hours, not minutes. When the ESXi host a SQL Server virtual
machine is running on experiences a hardware failure, vSphere HA detects this outage
and restarts the SQL Server virtual machine on another host. Based on the ACID
properties of the SQL database (discussed in section “vSphere App HA,” later in this
chapter) and the storage I/O crash-consistency properties within ESXi, the SQL Server
virtual machine is powered on, the Windows operating system boots, and SQL Server
loads and resumes operation. This is quite a handy feature if the failure occurs at 3 a.m.
Note
ESXi will maintain the correct order of writes to allow for a proper restart
after a crash. ESXi acknowledges a read or write to the guest operating
system only after the read or write is verified by the hardware controller to
ESXi. When reading the following KB article, be sure to note the
difference between a Type 1, bare-metal hypervisor (ESXi) versus a Type
2, hosted hypervisor (VMware Workstation):
http://kb.vmware.com/kb/1008542 . The exception is if vFRC is involved,
then reads originate from cache and writes are sent to storage.
The second benefit vSphere HA is the introduction of a new, cost-effective availability
option for the business that is not available in the physical world. If the application can
sustain a reboot (say, one to five minutes) and the business is okay with crash-consistent
copies, then vSphere HA is an effective solution to provide increased availability. The
database will protect itself according to ACID properties, but you should also consider
other tiers of the application stack.
But wait, there's more! vSphere HA does not just work at the physical host level.
vSphere HA also has the ability to monitor the guest operating system and applications
running inside the guest operating system. Disabled by default, but configurable,
vSphere HA has the ability to monitor the virtual machine for heartbeat traffic from
VMware tools, network traffic generated by the virtual NIC, and storage traffic
originating from the virtual SCSI adapter. If these three conditions are not present,
vSphere assumes the virtual machine has blue screened and can reboot the virtual
 
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