We know that database blocks are transferred between the nodes through the interconnect, aka cache fusion traffic. Common misconception is that packet transfer size is always database block size for block transfer (Of course, messages are smaller in size). That’s not entirely true. There is an optimization in the cache fusion code to reduce the packet size (and so reduces the bits transferred over the private network). Don’t confuse this note with Jumbo frames and MTU size, this note is independent of MTU setting.
In a nutshell, if free space in a block exceeds a threshold (_gc_fusion_compression) then instead of sending the whole block, LMS sends a smaller packet, reducing private network traffic bits. Let me give an example to illustrate my point. Let’s say that the database block size is 8192 and a block to be transferred is a recently NEWed block, say, with 4000 bytes of free space. Transfer of this block over the interconnect from one node to another node in the cluster will result in a packet size of ~4200 bytes. Transfer of bytes representing free space can be avoided completely, just a symbolic notation of free space begin offset and free space end offset is good enough to reconstruct the block in the receiving side without any loss of data.This optimization makes sense as there is no need to clog the network unnecessarily.
Remember that this is not a compression in a traditional sense, rather, avoidance of sending unnecessary bytes.
Parameter _gc_fusion_compression determines the threshold and defaults to 1024 in 220.127.116.11. So, if the free space in the block is over 1024 then the block is candidate for the reduction in packet size.
Test cases and dumps
From the test cases, I see that three fields in the block can be used to determine the free space available in the block. If you dump a block using ‘alter system dump datafile..’ syntax, you would see the following three fields:
fsbo=0x26 fseo=0x1b6a avsp=0x1b44
fsbo stands for Free Space Begin Offset; fseo stands for Free Space End Offset; avsp stands for AVailable free SPace;
It seems to me from the test cases that LMS process looks up these fields and constructs the buffer depending upon the value of avsp field. If avsp exceeds 1024 then the buffer is smaller than 8K ( smaller than 7K for that matter).