1 IP Fragmentation Settings
IP Fragmentation Settings
When the Greenplum Database interconnect uses UDP (the default), the network interface card controls IP packet fragmentation and reassemblies.
If the UDP message size is larger than the size of the maximum transmission unit (MTU) of a network, the IP layer fragments the message. (Refer to Networking later in this topic for more information about MTU sizes for Greenplum Database.) The receiver must store the fragments in a buffer before it can reorganize and reassemble the message.
The following sysctl.conf operating system parameters control the reassembly process:
OS Parameter Description net.ipv4.ipfrag_high_thresh The maximum amount of memory used to reassemble IP fragments before the kernel starts to remove fragments to free up resources. The default value is 4194304 bytes (4MB). net.ipv4.ipfrag_low_thresh The minimum amount of memory used to reassemble IP fragments. The default value is 3145728 bytes (3MB). (Deprecated after kernel version 4.17.) net.ipv4.ipfrag_time The maximum amount of time (in seconds) to keep an IP fragment in memory. The default value is 30.
The recommended settings for these parameters for Greenplum Database follow:
net.ipv4.ipfrag_high_thresh = 41943040
net.ipv4.ipfrag_low_thresh = 31457280
net.ipv4.ipfrag_time = 60
2 Networking
The maximum transmission unit (MTU) of a network specifies the size (in bytes) of the largest data packet/frame accepted by a network-connected device. A jumbo frame is a frame that contains more than the standard MTU of 1500 bytes.
Greenplum Database utilizes 3 distinct MTU settings:
– The Greenplum Database gp_max_packet_size server configuration parameter. The default max packet size is 8192. This
default assumes a jumbo frame MTU.
– The operating system MTU setting.
– The rack switch MTU setting.
These settings are connected, in that they should always be either the same,
or close to the same, value, or otherwise in the order of :
Greenplum < OS < switch for MTU size.
9000 is a common supported setting for switches, and is the recommended OS and rack switch MTU setting for your Greenplum Database hosts.