Interaction between databse cache, DBWR and datafiles

I'm trying to explain the mechanisms of the interaction between the buffer cache, dbwr and the datafiles to others and as often happens i've confused myself so that I no longer understand myself!

As I understand it the process works like this 1. there are two "lists" describing the buffers: the LRU list and the dirty list
2. when a request to access data is made (to read or write) the LRU list is searched from the LRU end up
3. if the relevant buffer is found it is moved to the MRU end, modified and marked dirty
4. if it isn't found, the block is read into the middle of the LRU list, or the MRU end, or the first free block from the LRU end depending on which document you read(?)
5. while it is searching for a free buffer any dirty buffers encountered are moved to the dirty list
6. If no free buffers are found by the threshold limit (or the timeout occurs), dbwr writes the buffers in the dirty list to disk 7. the newly found free buffer is written to and then moved to the MRU end and marked dirty therefore moving the older dirty buffers down the list

it is step 4 that most troubles me as the documentation seems to contradict itself

Also The other questions I can't answer by reading the documentation are:
* Is a buffer "block sized" ie, one block per buffer?

• when dbwr writes a dirty buffer to the datafiles does this buffer become empty or is the modified version moved back to the MRU end of the LRU list (or to the middle maybe, seeing as we are using 8i).
• If these written dirty buffers are emptied and therefore free, does this not contradict the purpose of keeping recently accessed buffers/blocks in memory
• Does "touching" a buffer mean 'searched' or 'modified'/'read'
• if another process wants access to a dirty buffer i think it is flagged as having "waiters" but i'm not sure what happens to the buffer - does it get written to disk and then read back in? does the second modification get made immediately so only one write occurs?

I think my problems stem for not understanding the difference between "clean" and "free" I would suppose free means empty and clean means full but matching the datafile block. Also I can't see the point in writing dirty buffers to disk in order to free them because then they are recently used but emptied (if that is indeed the case) Received on Thu Jan 30 2003 - 09:07:48 CST