Guidelines for using Microsoft System Center Virtual Machine Manager with HP StorageWorks Storage Mirroring (T2558-96337, June 2009)

Enabling compression
By enabling compression, you can reduce the amount of bandwidth needed to transmit Storage
Mirroring data. When compression is enabled, the data is compressed before it is transmitted from
the source. When the target receives the compressed data, it uncompresses it and then writes it to
disk. On a default Storage Mirroring connection, compression is disabled.
Because the files that should be included in a replication set can generate a significant amount of
data, you should enable compression for the connection. For more information about enabling
compression, see the HP StorageWorks Storage Mirroring user's guide . However, keep in mind that
the process of compressing data impacts processor usage. If you notice an impact on performance
while compression is enabled in your environment, either adjust to a lower level of compression, or
leave compression disabled.
Configuring memory usage
Storage Mirroring uses memory to queue operations and data on both the source and target. Since
the source server is typically running a production application, it is important that the amount of
memory Storage Mirroring and the other applications use does not exceed the amount of RAM in the
system. If the applications require more memory than there is RAM, the system will begin to swap
pages of memory to disk and the system performance will degrade.
Using SQL Server as an example, for instance, SQL Server will use all of the available system memory
when needed by default, and it may use almost all of the system memory during high-load operations.
These high-load operations are precisely what cause Storage Mirroring to need memory to queue the
data being changed by SQL Server. On a server with 1 GB of RAM running SQL Server and Storage
Mirroring, you might configure SQL Server to use only 512 MB and Storage Mirroring to use 256
MB, leaving 256 MB for the operating system and other applications on the system. Many other
server applications will use almost all system memory by default, so it is important to check and
configure applications appropriately, particularly on high-capacity servers.
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