Re: Chunk Writes

Maria Childress wrote: > Does anyone know what a chunk write is? Does a chunk write relate to > a chunk of disk on the OnLine system? Or does it relate to the buffer > queues? "Chunk Write" refer to the flushing dirty buffers in shared memory back to disk during the checkpoint. The "tbstat -F" will show you the statistics. The "chunk" refers to a piece of contiguous physical disk space, this term is used by Informix to represent physical disk space. The "tbcheck -ce -p" command output will give you the idea. > I think what is happening is that all modified pages in the buffers that > need to be written to disk are sorted by their disk chunk number. Then each > page cleaner can write all the modified pages for a given disk chunk in a > single I/O operation. Not exactly. During checkpoint, the dirty buffers are indeed sorted by their chunk number and location on chunks. But the page cleaner can't write all the dirty pages "in a single I/O operation". Reason: all the dirty pages belong to a chunk may not be contiguously located right next to each other on the disk chunk. Example:If you have page 1,2,3,5,8,9,10 dirty and page 4,6,7 clean; the page cleaner will do 2 big buffer writes and 1 normal buffer write to complete this chunk which means total of 3 "chunk write"s. > Hope this all makes sense, if not maybe we can discuss it. > > Thanks in advance for all input. Eric ========================================================================== Eric Chen Pyramid Technology email: echen@pyramid.com ==========================================================================