Storage Area Networks For Dummies®

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Par t V: Understanding the Cool Stuff
Many backup applications have versioning capability, which you may have
heard called incremental or differential backup. (See Chapter 9 for more information
on backup methods.) Some backup software (Tivoli Storage Manager
is a good example) always uses the versioning method to speed backup. You
do a full backup the first time, and from then on, only the changes are stored.
More intelligent file- and block-based monitoring solutions monitor data as it
is being stored or replicated, and only store or move the delta changes that
occur. A delta is an offset of the original data. As an example, take a Microsoft
Word document. Suppose that you create a document and save it.
That night, the document gets backed up with all the other files. The next
day, you reopen the same document, change a single word, and save it under
a different file name. The new file is a version of the first file because the files
are basically the same, but the new information in the document (the word
you changed) is a delta to the original. If there were a way to monitor the
blocks that stored the file as it was written to disk, you would know that the
file only changed slightly.
A file-compare de-dupe method would fail here because the file names are different.
A backup solution would back up the entire file again, because it looks
like a different file to the backup software than the original. Even a file-level
hashing solution would probably fail, because the file would be offset by the
new data and a new hash would occur. If you needed to replicate the file for
disaster recovery, you would also still need a lot of network bandwidth.
Only a delta versioning solution or a full sub-block-level hashing solution
would be able to de-duplicate this file:
✓ Block-level delta versioning: This method works by monitoring updates
on disk at the block level and storing only the data that changed in relation
to the original data. Block-level delta versioning is how snapshots
work. Each snapshot contains only the changes to the original data.
Block-level delta versioning can also reduce data replication requirements
for disaster recovery (DR) purposes. Suppose that your company
wants to keep the remote data up to date every 6 hours, so you have to
replicate changes to the DR location every 6 hours. If a block of data on
disk at the local site is updated hundreds of times during the time delta
between the last replication and the new one, using a delta versioning
solution, only the last update to the block needs to be sent. Delta versioning
can greatly reduce the amount of data traveling from the local
site to the DR site.
✓ Sub-block-level delta versioning: This method is the same as the block
method, but works at the byte level and can be many times more efficient
in reducing duplicate data than even block-level versioning. This type of
versioning is also known in the industry as microscanning (see Figure 13-7).