HP Storage Mirroring V5.1 Software - Manual (Page 186)

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Maximum system memory for queue—This is the amount of Windows system memory, in MB,
that will be used to store data in queues. When exceeded, queuing to disk will be triggered. This
value is dependent on the amount of physical memory available but has a minimum of 32 MB. By
default, 128 or 512 MB of memory is used, depending on your operating system. If you set it
lower, Storage Mirroring will use less system memory, but you will queue to disk sooner which
may impact system performance. If you set it higher, Storage Mirroring will maximize system
performance by not queuing to disk as soon, but the system may have to swap the memory to
disk if the system memory is not available.
Since the source 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 are configured to use more memory than there is RAM, the
system will begin to swap pages of memory to disk and the system performance will degrade.
For example, by default SQL Server will use all of the available system memory when needed,
and this may happen during high-load operations. These high-load operations cause Storage
Mirroring to need memory to queue the data being changed by SQL Server. In this case, you
would need to configure the applications so that they collectively do not exceed the amount of
RAM on the server. Perhaps on a server with 1 GB of RAM running SQL Server and Storage
Mirroring, you might configure SQL Server to use 512 MB and Storage Mirroring to use 256 MB,
leaving 256 MB for the operating system and other applications on the system. Many server
applications default to using all available system memory, so it is important to check and
configure applications appropriately, particularly on high-capacity servers.
Any changes to the memory usage will not take effect until the Storage Mirroring service has
been restarted on the server.
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Maximum disk space for queue—This is the maximum amount of disk space, in MB, in the
specified Folder that can be used for Storage Mirroring disk queuing, or you can select Unlimited
which will allow the queue usage to automatically expand whenever the available disk space
expands. When the disk space limit is reached, Storage Mirroring will automatically begin the
auto-disconnect process. By default, Storage Mirroring will use an unlimited amount of disk space.
Setting this value to zero ( 0 ) disables disk queuing.
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Minimum Free Space—This is the minimum amount of disk space in the specified Folder that
must be available at all times. By default, 50 MB of disk space will always remain free. The
Minimum Free Space should be less than the amount of physical disk space minus Maximum
disk space for queue.
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Alert at following queue usage percentage—This is the percentage of the disk queue that
must be in use to trigger an alert message in the Windows Event Viewer. By default, the alert will
be generated when the queue reaches 50%.