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Once the original volume is replicated to the remote site, regular closely spaced snapshots
are scheduled on the primary production server; the replica server is then updated
snapshot at a time. There are two methods of implementing snapshots: one is the
conventional Copy-On-Write (COW) and the other is a more advanced technique called
Redirect-On-Write (ROW). AMI implements ROW in order to offer the best
performance and space efficiency. However, irrespective of the method in use, at the
core these implementations track and maintain this difference data in granular regions
called "chunks". Typically, these chunks are 64KB in size. The storage stacks also
maintain a list of delta chunks indicating the difference in contents between successive
snapshots. This permits replication of only the delta chunks in successive snapshots.
Normally, during write operations in a snapshot enabled environment, sub-chunk sized
writes result in read-modify-write operations, meaning that any such small I/Os will
generate a large number of sub-chunk granular duplicates. A close observation of the
types of I/Os generated by various application servers reveals that these types of small,
duplicate I/Os make up the majority. For example, SQL servers generate random I/Os of
8K blocks, and Exchange servers generate random I/Os of 4K blocks, making it easy to
see that even when chunk sized I/Os are replicated, they may carry multiple packets of
duplicated data. To remedy this, the deduplication feature in StorTrends iTX utilizes a
very I/O-efficient algorithm to detect duplicate data and eliminates them, to further
reduce the amount of data transported during asynchronous replication. The screenshot
below demonstrates the amount of data reduction achieved in a typical real-world
asynchronous replication scenario.
Figure 6: Deduplication and WAN Transfer Acceleration Management in StorTrends iTX
Naturally, this data reduction is achieved at the expense of generating more read I/Os and
subsequent byte-level comparisons. These extra chores are performed in order to
eliminate duplicate data that has been already transported in previous cycles. It must be
noted that here data deduplication is employed only for data transfer optimizations over
WAN. In such replication environments this is very beneficial as it curtails a lot of
replication time resulting in faster replications and thereby yielding excellent RPO.
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StorTrends/ManageTrends (version 2.7) Web Interface User's Guide for the StorTrends 2200 Storage Appliance
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