Storage Area Networks For Dummies®

Chapter 6: SANs and Disaster Recovery
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When you calculate what downtime will cost your business, you also need to
factor in these two considerations:
✓ Recovery Time Objective (RTO): This factor is the amount of time that
it takes to get your systems back online. If you have a disaster, how
long would it take you to get all your applications up and running again
at another site? In most cases today, that answer is way too long. Most
businesses back up their data to tapes and ship the tapes to an off-site
location. Recovering the data from tapes back onto disks takes too long.
This is why companies are consolidating their critical data into a SAN
with the capability to replicate in real time to a recovery site. By using
a SAN to replicate blocks of data to a remote site for disaster recovery,
your data is already on disk at the DR site and ready to use if your primary
site goes away.
✓ Recovery Point Objective (RPO): This is the last consistent data transaction
before the disaster. If you had a disaster, how much data would be
lost? In most cases today, that answer is way too much. Most companies
today can’t afford to lose any data. What if the data they lost was the
record of your last mortgage payment or that huge stock purchase you
made just before the stock went into the stratosphere? You wouldn’t like
that very much, I’m sure. That is why using real-time data copy with an
intelligent SAN makes sense for many companies. Using SAN technology
as a basis for disaster recovery allows you to recover more data faster.
Gathering the data for a
disaster-recovery plan
Creating the building blocks of a disaster-recovery solution using a SAN
requires some thought on your part. In a nutshell, the questions to address
are the following:
✓ Data capacity and change rate: How much data do I need to copy?
You should have to copy all your data to your disaster site only once.
After the bulk of the data is there, you need to copy only the data that
changes from day to day. This means that you are concerned about only
the data that you write on a daily basis, not the data you read. You need
to determine what percentage of your data changes every day, not the
total amount of data you have.
You can determine your daily total write rate by finding out the size of
your daily incremental backups. Incremental backups deal with only the
data that was changed between backups.