Datasheet

overvieW of availabilitY MechanisMs
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Undeniably, asynchronous host-based replication solved a real problem for Windows admin-
istrators for nearly 10 years. In fact, it solved two problems:
Data
•u protection in the sense that data could be “protected” (replicated) out of the production
server more often than nightly, which is where tape is limited
Data
•u availability in the sense that the secondary copy/server could be rapidly leveraged if the
primary copy/server failed
Asynchronous replication addressed a wide majority of customers who wanted to better pro-
tect their data, rather than making nightly tape backups, but who could not afford to implement
synchronous storage arrays. We will cover asynchronous replication later in this book. For now,
note that as a file system–based mechanism, asynchronous replication on its own is a category
of data protection that is arguably diminishing as the next two technologies begin to flourish:
Clustering and Asynchronous Replication.
Clustering
Ignoring the third-party asynchronous replication technologies for a moment, if you were a Microsoft
expert looking at data protection in the early days of Windows Server, your only choice for higher
availability was redundancy in the hardware through network interface card (NIC) teaming, redun-
dant power supplies and fans, and of course, synchronous storage arrays. When the synchronous
arrays are used for availability purposes, we must remember that hardware resiliency only addresses
a small percentage of why a server fails. For the majority of server and service outages that were soft-
ware based, Microsoft originally addressed this with Microsoft Cluster Services (MSCS) and other
technologies that we’ll cover later in this book.
MSCS originally became available well after the initial release of Windows NT 4.0, almost like
an add-on or more specifically as a premium release with additional functionality. During the
early days of Windows clustering, it was not uncommon for an expert-level Microsoft MCSE or
deployment engineer (who might be thought of as brilliant with Windows in general) to struggle
with some of the complexities in failover clustering. These initial challenges with clustering were
exacerbated by the first generation of Windows applications that were intended to run on clus-
ters, including SQL Server 4.21 and Exchange Server 5.0. Unfortunately, clustering of the applica-
tions was even more daunting.
In response to these challenges with the first built-in high availability mechanisms, many of
the replication software products released in the mid-1990s included not only data protection
but also availability. Initially, and some still to this day, those third-party replication technolo-
gies are burdened by support challenges based on how they accomplish the availability. But in
principle, they work by either extending the Microsoft clustering services across sites and appre-
ciable distances but allowing the cluster application to handle the failover. Or they use a propri-
etary method of artificially adding the failed server’s name, IP, shares, and even applications to
the replication target and then resuming operation. The industry leader in asynchronous rep-
lication is Double-Take from Double-Take Software, formerly known as NSI Software. Another
example of this technology is WANSync from Computer Associates, acquired from XOsoft.
XOsoft provided the initial WANSync for Data Protection, and followed up with WANSync
HA
,
which included data availability. We will discuss these products in Chapter 3.
MSCS continued to evolve and improve through Windows 2000, Windows Server 2003, and
Windows Server 2003 R2. That trend of continued improvement would continue through the more
recent Windows Server 2008 and the newly released Windows Server 2008 R2. But that isn’t the
whole story. MSCS will be covered in Chapter 6.
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