Storage Area Networks For Dummies®

Chapter 8: Networking SANs
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which is your disaster recovery site. You want to use the link to transmit data
in real time from New York to Boston for online backup of your mail servers.
You can’t afford to buy a large tape library for both locations, so only the
Boston location has been outfitted with the expensive tape library. You what
to know what kind of link you need to lease between those locations so that
your remote backup solution works.
To figure this out, you need to know how much data is changed in the e-mail
sever on a daily basis at the New York site. Because only write data (data
that’s written to disk) needs to be copied across the link, you don’t need to
worry about data that is only read from your disks.
If you use the rule that every megabyte of write data written per second would
require a 10-megabit-per-second network link to copy the data across, you can
easily calculate the bandwidth requirement for your remote backup solution.
Suppose that on a daily basis, all the e-mail users in New York change between
56GB and 58GB of data per day. These changes occur at about the same rate
during a normal eight-hour workday. This would mean that you need a network
link that can handle a peak rate of 2 megabytes per second, which, using
the rule above, would equal a 20 Mbps link, which is about half a T-3 link.
Here how to calculate this:
2 megabytes per second × 60 seconds = 120 megabytes per minute
120 megabytes per minute × 60 minutes = 7.2 gigabytes per hour
7.2 gigabytes per hour × an 8-hour day = 57.6 gigabytes per day
You can determine that you need at least half a T-3 link to accommodate your
requirements. T-3s can be expensive when leased on a monthly basis. Psst —
wanna save your company some monthly expenses by using a cheaper leased
line? You could either put limits on everyone’s e-mail to make sure that they can
store only half the amount of data, or use a SAN extender device that provides
compression. If your extender could use two-to-one compression (2:1), the size
of the transmitted data would be halved. This means that instead of leasing half
a T-3 link, you could get away with a standard 10BaseT Ethernet link.
20/2 = 10-megabit link
Compressing your data before transmitting it over the network is a great way
to save money in leased line expenses. When you choose your SAN extension
device, make sure the vendor supports data compression in the device.