Re: Logical log write volumes

Malcolm Weallans (onlinedbc@cix.compulink.co.uk) wrote: : Piggy-back COMMITs are a real event. All this means is that the logical : log buffer in shared memory is flushed to disk periodically. If a second : commit occurs before the log buffer is flushed it will continue flushing : the log buffer to disk. This does not alter the amount of data written : to the log, That strikes me as a bit of a fuzzy explanation, Malcolm. During Unbuffered Logging each commit causes an immediate Log Buffer flush and so a second commit could not occur "before the log buffer is flushed". However, there is a certain amount of work involved in preparing the buffer flush, and if subsequent commits arrive during this preparation, they are included in the same buffer flush rather than being redirected to the next Log Buffer in sequence, thereby causing an immediate flush of the next (presumably almost empty) Log Buffer. One would therefore expect to see less wastage, and therefore slower log consumption than would be experienced with Unbuffered Logging without Group Commit. During Buffered Logging a buffer flush does not occur until the buffer fills up (ignoring special cases like checkpoints), and therefore the concept of Group Commit does not apply, as no more data could be included in the buffer anyway. In this scenario, there should be no space wastage, and I would therefore expect to see slower log consumption than with Buffered Logging, for a given transactional workload. Although Malcolm is correct in that the number of bytes of data written to the log would be the same, I believe the number of pages consumed would be different. -- graeme -- Disclaimer: The author's opinions are his own, and not necessarily those of Pyramid Technology Ltd. or Pyramid Technology Inc. --------------------------------------------------------------------------- -m------- Graeme Sargent Voice: +44 (0)252 373035 ---mmm----- Senior Database Consultant Fax : +44 (0)252 373135 -----mmmmm--- Pyramid Technology Ltd. Telex: Tell who??? -------mmmmmmm- Farnborough, Hants GU14 7PL Email: graeme@pyra.co.uk --------------------------------------------------------------------------- We have the technology. The tricky bit is learning how to use it.