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Intraday Data Protection
Intraday data protection allows data to be protected multiple
times per day. The copy is often made to the same disk subsystem or to a
separate disk subsystem depending upon the technology. Intraday data protection
allows data that is lost near the end of the day to be restored from a point in
time more recent than the previous night’s backup tapes. For example, if a SQL
database is corrupted at 3:30 p.m. and an intraday copy was created at 12:00
a.m., then only 3.5 hours of data would be lost. Without intraday data
protection, restores would be performed using the data from the last nightly
backup, which for this example, could represent as much as 7.5 hours of lost
data assuming the work day began at 8:00 a.m.
In addition, since restores are typically from a disk storage
system, not tape, recovery time is measured in minutes, not tens of minutes to
hours with tape.
There are 3 basic types of intraday data protection:
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Snapshots are available on certain disk storage subsystems
and file systems. Snapshot software either makes or retains historical
versions of files on a frequency that is determined by the user. Snapshots
can be scheduled as often as once an hour. Snapshots are limited to a
fixed maximum number, typically 64 or 256, depending on the implementation.
Once this maximum number is reached, the oldest snapshot is eliminated when
a new snapshot is taken. Snapshots provide for intraday data restores,
assuming that the deletion or corruption happened within the period in which
a snapshot is available. If the deletion or corruption happened earlier than
the oldest snapshot, a restore would have to be performed from traditional
backup tapes. Also, if the primary disk system fails, not only is all of the
primary data lost, but so are all of the snapshots. In both of these
cases, traditional nightly backups would have to be used to restore the lost
or corrupted data.
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Application dumps are available for certain applications,
such as Microsoft SQL Server and Oracle (Recovery Manager – RMAN), which
have a backup utility built into them. With application dumps, point
in time database images can be created multiple times during the day to
reduce the exposure to losing more than a few hours of data in the event of
system failure or database corruption.
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Continuous Data Protection (CDP) is fairly new and not widely
deployed at this time. Think of CDP as continuous snapshots that can also be replicated to another
system, resulting in a historical record of data that is up to date anywhere
from within seconds to an hour. With CDP, if a disk subsystem fails or
becomes corrupted at 12:25 p.m., a restore could be performed from the CDP
data just seconds to minutes earlier resulting in minimal data loss. As with
all technology, there is a trade off - CDP creates a byte-level replicated
copy and therefore takes additional system, network and storage resources.
As with other forms of intraday data protection, data that is protected by
CDP must also be backed up nightly with traditional backup applications.
Snapshots, application dumps and CDP are not replacements for
weekend and nightly backups. In each case, their function is to keep a
minimum number of intraday copies to reduce the amount of data lost in the event
of midday data loss or corruption. In addition, weekend and nightly backups are
still required for long term history.
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