ExpressCluster X 2.0 for Windows Reference Guide - Nec

Section II Resource details
Understanding mirror disk resources
Mode
Switch the synchronization mode of mirroring.
Mode Overview Explanation
Synchronous Complete match of the Writing the data to the mirrored disk is finished
data in the active and when writing the data to both local and remote
standby servers is
guaranteed.
disks is finished.
Asynchronous The order to write in the
updated data is
guaranteed. However, the
latest updated data may
be lost, if a failover is
performed in the state that
a mirror disk resource
cannot be deactivated as
servers are down.
Writing the data to the mirrored disk is finished
when writing the data to the local disk is
finished.
The data is transferred to the remote disk after
writing request is queued and performed on
the background.
After queuing is kept in the kernel space
memory, it is transferred to the user space
memory. When the volume of data reaches a
limit that the user space memory can keep, the
data is sent out to a temporary file and kept
there.
Kernel Queue Size
Specify the size of the request for writing to the remote disk to be kept in the kernel space
memory when the mode is set to “Asynchronous.” Normally, default value is specified.
Input and output are completed, if writing data can be saved in the kernel queue.
If taking data into the application queue is delayed as the load on CPU is high, the size is set
larger. However, if the size is too large, it will result in compressing the system resource.
Application Queue Size
Specify the size of the request for writing to the remote disk to be kept in the user space memory
when the mode is set to “Asynchronous.” Normally, the default value is used. However, if a
high-speed network is used, the frequency of creating a temporary file can be reduced and the
overhead caused by input and output can be decreased by making the queue size larger.
Upper Bound of Communication Band
When the mode is” asynchronous”, the server tries to transfer data that has been queued to the
standby server. When the channel for mirror connection is used for connections for other
applications, the communication band may become busy. In this case, by setting the bound of
communication band for the mirror connect communication, the impact on other communications
can be reduced. If the communication band for mirror connect is smaller than the average amount
of data to be written into the mirror disk, the queued data cannot be fully transferred to the
standby server, which can result in overflow and suspension of mirroring. The bandwidth should
be large enough for data to be written into the business application.
This function makes a limit to the communication band by having a maximum of one-second
pause when the total amount of data to be transferred per second exceeds the configured value. If
the size of data to be written into the disk at a time is greater than the configured value, expected
performance may not be achieved. For example, even if you set the value of communication band
limit to be 64Kbyte or smaller, the actual amount of communication during copy can be greater
than the configured value because the size of data to be transferred for a copy of a mirror disk at
a time is 64 Kbyte.
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