Datasheet

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CHAPTER 1 What Kind of Protection do You need?
be assured 24/7 availability, including access on premises or from any Internet location.
Even within one company, and for a single application, the protection method will differ.
As an interesting twist, if the company we are discussing is Amazon.com, where their
entire business is driven by shipping, that might be the most mission-critical department
of all. Microsoft Exchange provides four different protection methods even within itself,
not including array mirroring or disk- and tape-based backups (more on that in Chapter 7).
Similarly, Microsoft SQL Server might be pervasive across the entire range of servers in the •u
environment — but not every database may warrant mirroring, clustering, or replication at all.
If the data protection landscape was a graph, the horizontal X axis could be defined as a data
loss, starting at 0 in the left corner and extending into seconds, minutes, hours, and days as we
move across the graph. In short, what is your recovery point objective (RPO)?
We’ll cover RPO and cost in Chapter 2. For now, know that RPO is one of the four universal
metrics that we can use to compare the entire range of data protection solutions. Simply stated,
RPO asks the question, “How much data can you afford to lose?”
In our rhetorical question, the key verb is afford. It is not want — nobody wants to lose any
data. If cost was not a factor, it is likely that we would all unanimously choose zero data loss as
our RPO. The point here is to recognize that even for your mission-critical, or let’s just say most
important, platforms, do you really need synchronous data protection — or would asynchronous
be sufficient?
Should You Solve Your Availability Need with
Synchronously Replicated Storage?
The answer is that “it depends.” Here is what it depends on: If a particular server absolutely, posi-
tively cannot afford any loss of data, then an investment in synchronously mirrored storage arrays
is a must. With redundancy within the spindles, along with two arrays mirroring each other, and
redundant SAN fabric for the connectors, as well as duplicated host bus adapters (HBAs) within the
server to the fabric, you can eliminate every SPOF in your storage solution. More importantly, it is
the only choice that can potentially guarantee zero data loss.
This is our first decision question to identify what kinds of availability solutions we should
consider:
If we really need “zero data loss,” we need synchronously mirrored storage (and additional
•u
layers of protection too).
If we can tolerate anywhere from seconds to minutes of lost data, several additional technolo-
•u
gies become choices for us — usually at a fraction of the cost.
sy n c h r o n o u s v s . As y n c h r o n o u s
Synchronous versus asynchronous has been a point of debate ever since disk mirroring became
available. In pragmatic terms, the choice to replicate synchronously or asynchronously is as sim-
ple as calculating the cost of the data compared with the cost of the solution. We will discuss this
topic more in Chapter 2, as it relates to RPO and return on investment (ROI), but the short version
is that if the asynchronous solution most appropriate for your workload protects data every 15
minutes, then what is 15 minutes’ worth of data worth?
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