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In an environment environments where the replication bandwidth band is a variable band (WAN replication environment), the replication link can sometimes become congested. Because of this, if This causes the primary node's I/O waits, the performance of the to wait, resulting in a performance degradation of local I/O will be degraded, which is undesirable. When detecting this congestion, you can configure it to suspend replication. Instead, in the situation where replication is interrupted, the primary data set is ahead of the secondary data, and these advanced data blocks are recorded as out-of-sync (OOS). when congestion is released, after all these oos is resolved through resynchronization. The following is an example of setting the congestion policy.. Congestion mode is a configuration to respond to this situation.

When congestion is detected, replication is suspended and buffered data is slowly sent to the target while logging local I/O to OOS. During this process, the primary is in an Ahead data state compared to the secondary, and once it finishes sending buffered data, it automatically enters sync mode to synchronize the OOS areas that failed to replicate.

Here is an example of setting up a congestion policy

In the resource configuration file, set the congestion mode with the on-congestion option item sets and set the congestion mode, and detection threshold with the congestion-fill item sets the recognition threshold for congestion.

resource <resource> {
  net {
    sndbuf-size 20M1G;
    on-congestion pull-ahead;
    congestion-fill 2G900M;
    congestion-extents 20005500;
    ...
congestion-highwater 20000;
 }   ...
  }
  ...
}

The pull-ahead option is used with congestion-fill and , congestion-extents, or congestion-highwater. The recommended values for each property are as follows

• Set congestion-fill

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• When linking to approximately 90% of the size of sndbuf-size. If you are integrating a replication accelerator (DRX), set it to about 90% of the DRX buffer size.If DRX is not linked, set to 90% of sndbuf-size. However, if the buffer is allocated a large size, say 10GB or more, the 90% threshold may be too large to detect congestion, so this should be adjusted to a reasonable value through tuning.

• The recommended value for congestion-extents is 90% of the al-extents setting.

• congestion-highwater detects congestion based on packet count. It is particularly appropriate for use in DR environments where capacity-based detection of replication congestion is not suitable. It is set to 20000 by default and is enabled by default. It is disabled when set to 0 and has a maximum value of 1000000.

Disk flush

If the target node suddenly goes down due to power failure during replication, data loss may occur if the disk cache area is not backed up by a battery backup device (BBWC). In order to prevent this in advance, in the process of writing data to the disk of the target, after data is written to the media, the flush operation is always performed to prevent data loss.

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