Data Buffer Settings

NET_BUFFER_SZ, SIZE_DATA_BUFFERS and NUMBER_DATA_BUFFERS - how they work and how to configure them

Details:
Here is some information on undocumented features for setting the Network Buffer Size, Data Buffer Size, and Number of Data Buffers used by NetBackup (tm) on UNIX platforms.

NOTE: These settings are used to tune the performance of NetBackup.


TCP/IP Network Buffer Size.

/usr/openv/netbackup/NET_BUFFER_SZ is a file containing a number indicating the TCP/IP socket buffer size that should be used for data transfers between the NetBackup media server and its clients. If the file does not exist, the default value used is 32032 bytes.

The default value can be changed by creating the file /usr/openv/netbackup/NET_BUFFER_SZ on the NetBackup media server and client systems.

The /usr/openv/netbackup/NET_BUFFER_SZ file is a text file containing a single line specifying the value of the Network Buffer Size in bytes in decimal format.

For example, a Network Buffer Size of 64KB could be configured and checked like this:

# echo "65536" > /usr/openv/netbackup/NET_BUFFER_SZ
# cat /usr/openv/netbackup/NET_BUFFER_SZ
65536
#

Specifying a larger value in the NET_BUFFER_SZ file may improve the performance of backups and restores. It is recommended that if NET_BUFFER_SZ is used, the same value should be set on all the NetBackup media servers and clients.



Data Buffer Size and Number of Data Buffers.

The NetBackup media server uses shared memory to buffer data between the network and the tape drive (or between the disk and the tape drive if the NetBackup media server and client are the same system). By default, NetBackup uses a default value of 8 x 32KB shared memory buffers for non-multiplexed backups and 4 x 64KB for a multiplexed backup.

These buffers can be configured by creating the files /usr/openv/netbackup/db/config/SIZE_DATA_BUFFERS and /usr/openv/netbackup/db/config/NUMBER_DATA_BUFFERS on the NetBackup media server. These parameters are meant exclusively for media servers, and should not be used on a pure master server or client.

The SIZE_DATA_BUFFERS file should contain a single line specifying the data buffer size in bytes in decimal format.
The NUMBER_DATA_BUFFERS file should contain a single line specifying the number of data buffers in decimal format.

The value in the SIZE_DATA_BUFFERS file has to be a multiple of 1024B.
It is common to increase SIZE_DATA_BUFFERS to 256KB and NUMBER_DATA_BUFFERS to 16.


IMPORTANT: Because the data buffer size equals the tape I/O size, the value specified in SIZE_DATA_BUFFERS must not exceed the maximum tape I/O size supported by the tape drive or operating system. This is usually 256 KB or 128 KB.


In order to configure NetBackup to use 16 x 256 KB data buffers, specify 262144 (256 x 1024) in SIZE_DATA_BUFFERS and 16 in NUMBER_DATA_BUFFERS:

# echo "262144" > /usr/openv/netbackup/db/config/SIZE_DATA_BUFFERS
# echo "16" > /usr/openv/netbackup/db/config/NUMBER_DATA_BUFFERS
# cat /usr/openv/netbackup/db/config/SIZE_DATA_BUFFERS
262144
# cat /usr/openv/netbackup/db/config/NUMBER_DATA_BUFFERS
16
#

Care should be taken when changing these values. For example, increasing the tape buffer size can cause some backups to run slower and there have been cases where there have been restore problems.


IMPORTANT: After making any changes, it is vitally important to verify that the following tests can be completed successfully:

1. Run a backup
2. Restore data from the new backup created in step 1
3. Restore data from a backup created prior to the changes to SIZE_DATA_BUFFERS and NUMBER_DATA_BUFFERS


The number and size of the data buffers used by the NetBackup media server can be checked by looking in the bptm log file:

12:02:55 [28551] <2> io_init: using 32768 data buffer size
12:02:55 [28551] <2> io_init: CINDEX 0, sched bytes for monitoring = 2000
12:02:55 [28551] <2> io_init: using 8 data buffers

or

15:26:01 [21544] <2> mpx_setup_restore_shm: using 12 data buffers, buffer size is 65536


Increasing the size and number of the data buffers will use up more shared memory, which is a limited system resource. The total amount of shared memory used is: (buffer_size * num_buffers) * drives * MPX.

So, for two tape drives, each configured with an MPX of 4, and with 16 data buffers of 256 KB, the total shared memory usage would be:

(16 * 256k) * 2 * 4 = 32 MB (32768 KB)

Take into consideration the total system resources and the entire network.
The Maximum Transmission Unit (MTU) for the LAN may also have to be changed.


NOTE: For Digital Linear Tape (DLT) drives, the number that seems to give the best performance for SIZE_DATA_BUFFERS is 65536. Further performance improvements can be achieved by increasing the value in the NUMBER_DATA_BUFFERS file.