Veritas NetBackup™ Appliance Fibre Channel Guide

About the Fibre Transport chunk size

The chunk size specifies the amount of data that is buffered before transmission over Fibre Transport (FT). It is an advanced setting on the sender side for optimized duplication and replication to other NetBackup appliances over FT.

By default, the chunk size is not set or used. The chunk size setting is not required for optimized duplication and replication over FT. It is highly dependent on user deployment and network status. If you find it necessary to improve backup performance, you can adjust the chunk size as needed to determine a proper value for your environment and the current network status.

Typically, the sender on the initiating appliance receives a block of data and starts the transmission regardless of its size. After this data block is transmitted, the sender waits for a confirmation from the receiver (or target appliance) before it transmits the next block. The transmission continues if the appliance receives confirmations without interruption. If the network latency is high, confirmation time also increases.

By setting the chunk size, you determine that data blocks must be delivered by using chunks of the specified size. In a single transmission session, if the sender receives a block of data that is greater than the specified chunk size, it fragments the data into chunks for transmission. The remaining data from the fragmentation is allocated to the next chunk. If a data block is smaller than the specified chunk size, the sender adds it into the buffer to link it together with data from the preceding or the following block. Data block linking continues until the buffered data has reached the chunk size, and then the appliance transmits the chunk. After transmitting, the sender waits for a confirmation from the receiver before it sends out the next chunk. If the session closes, the remaining data is transmitted.

A data block over FT is typically 128 KB or smaller. Setting the chunk size to a value equal to or greater than 128 KB allows the appliance to link together small data blocks into big chunks. Therefore, fewer confirmations are needed. The result may improve the transmission efficiency when the network latency is high. The data block linking also reduces the overhead that is generated with the data.

Network latency can be high in the following scenarios, and you can consider using the chunk size to see if anything improves.

The default value for chunk size is "0", which means the chunk size is not set or used. If you set the chunk size to a value equal or greater than "1", you specify the size of each data chunk that is sent to the FC network. To enable and manage the chunk size setting, use the Main > Support > FibreTransport > ChunkSize command from the NetBackup Appliance Shell Menu. The available chunk size range is 1-2048 KB. The recommended values are 128 KB, 256 KB, 512 KB, 1024 KB, or 2048 KB. You may also need to test with other values to determine a proper value for your environment.

The chunk size may not work for the following reasons: