Datasheet

overvieW of availabilitY MechanisMs
|
7
If the overall business impact of losing those 15 minutes’ worth of data (including both lost
information and lost productivity) is more expensive to the business than the cost of a mirrored
and synchronous solution, then that particular server and its data should be synchronously mir-
rored at the storage level. As I mentioned earlier, the vast majority of corporate environments
cannot justify the significantly increased cost of protecting those last (up to) 15 minutes of lost
data — and therefore need an asynchronous protection model.
If your RPO truly and legitimately is zero, synchronously mirrored arrays are the only data
protection option for you, or at least for that particular application, on that particular server, for
that particular group within your company. To paraphrase a popular US television commercial
tagline: “For everything else, there’s asynchronous.”
Asynchronous Replication
Even in environments where one platform demands truly zero data loss and therefore synchro-
nous storage, the likelihood is that the remaining platforms in the same company do not. Again,
the statistics will vary, but recall the extremes described in the previous sections: 0.4 percent of IT
environments can cost-justify synchronously mirrored storage but only 1 percent of environments
can rationalize half a business day of data loss with typically 1.5 days of downtime. If those statis-
tics describe both ends of the data protection spectrum, then 98.6 percent of IT environments need
a different type of data protection and/or availability that is somewhere in between the extremes.
In short, while the percentages have likely changed and though your statistics may vary, most IT
environments need better protection than nightly tape, which is less expensive than synchronous
arrays.
In the Windows space, starting around 1997, the delivery of several asynchronous solutions
spawned a new category of data protection and availability software, which delivered host-based
(running from the server, not the array) replication that was asynchronous.
Asynchronous replication, by design, is a disk-to-disk replication solution between Windows
servers. It can be done throughout the entire day, instead of nightly, which addresses the main-
stream customer need of protecting data more frequently than each night. To reduce costs, asyn-
chronous replication software reduces cost in two different dimensions:
Reduced Telecommunications Costs Synchronous mirroring assures zero data loss by writ-
ing to both arrays in parallel. The good news is that both of the arrays will have the same data.
The bad news is that the servers and the applications could have a delay while both disk trans-
actions are queued through the fabric and committed to each array. As distance increases, the
amount of time for the remote disk to perform the write and then acknowledge it increases
as well. Because a disk write operation is not considered complete until both halves of the
mirror have acted on it, the higher-layer OS and application functions must wait for the disk
operation to be completed on both halves of the mirror. This is inconsequential when the two
arrays are side by side and next to the server. However, as the arrays are moved further from
the server as well as from each other, the latency increases because the higher-layer functions
of the server are waiting on the split disks. This latency can hinder the production application
performance. Because of this, when arrays are geographically separated, companies must pay
significant telecommunications costs to reduce latency between the arrays. In contrast to that,
asynchronous replication allows for the primary disk on the production server to be written to
at full speed, whereas the secondary disk has a replication target and is allowed to be delayed.
As long as that latency is acceptable from a data loss perspective, one can be several minutes
apart between the two disks and the result is appreciably reduced telecommunications costs.
572146c01.indd 7 6/23/10 5:42:20 PM