Michael Ludwig schrieb am 11.05.2010 um 00:24:59 (+0200):

> On page 382 of Helen's book, in the section "Preventing Slowdown on
> Batches", I'm reading the following:
>
> Two behaviours can cause huge batches to get progressively
> slower during executino.
>
> * [maintaining indexes - okay]
>
> * The accumulation of *back versions*, the deltas of the
> old rows that are marked for obsolescence once the
> update or delete operations are committed. Back version
> do not become available for garbage collection until the
> transaction completes. In transactions involving back
> version numbering in the tens to hundreds of thousands
> (or more), garbage colllection may never win its battle
> to clean out obsolete version. Under these conditions,
> only a backup and restore will sanitize the database
> completely.
>
> Not sure I'm following. I thought that once the huge batch is done and
> all transactions that are older than the batch completion have also
> completed, the back versions will have become eligible for garbage
> collection. Correct?
>
> If so, how will GC never win the battle?

I was reminded of this issue when I was reading a document stating that
*sweeping* may not have chances to finish its work during high database
load (section "When sweep can't do its work").

IBSurgeon Tips&Tricks: What you can see at Summary page in IBAnalyst?
http://www.scribd.com/doc/19815933/IBSurgeon-TipsTricks-What-you-can-see-at-Summary-page-in-IBAnalyst

--
Michael Ludwig