Thread

1. have SLRU truncation use callbacks

   Alvaro Herrera <alvherre@alvh.no-ip.org> — 2011-09-29T15:58:41Z

   Hi,
   
   Currently, the mechanism that SLRU uses to truncate directory entries is
   hardcoded in SlruScanDirectory.  It receives a cutoff page number and a
   boolean flag; segments found in the SLRU directory that are below the
   cutoff are deleted -- iff the flag is true.
   
   This is fine for most current uses, with one exception, but it is a bit
   too ad-hoc.  The exception is the new NOTIFY code (async.c); on
   postmaster (re)start, that module wants to simply zap all files found in
   the directory.  So it forcibly sets a different "pagePrecedes" routine,
   then calls the truncation procedure with a made-up cutoff page.
   
   Also, clog.c has a check that it scans the directory to figure out if
   any segments would be removed, were we to do the thing for real.  (This
   is so that it can skip WAL flush in case none would).
   
   (Now, this code is also getting in the way of some changes I want to do
   on multixact truncation for the keylocks patch; hence the motivation.
   But I think these changes stand on their own, merely on code clarity
   grounds).
   
   So what I propose is that we turn SlruScanDirectory into a mere
   directory walker, and it invokes a callback on each file.  We provide
   three callbacks: one that simply removes everything (for NOTIFY); one
   that removes everything before a given cutoff page; and a simple one
   that reports "is there any removable file given this cutoff", for clog.c
   uses.
   
   Patch is attached.
   
   Opinions?
   
   -- 
   Álvaro Herrera <alvherre@alvh.no-ip.org>
   

2. Re: have SLRU truncation use callbacks

   Kevin Grittner <kevin.grittner@wicourts.gov> — 2011-09-29T17:51:38Z

   Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
    
   > But I think these changes stand on their own, merely on code
   > clarity grounds).
    
   After a quick scan, I think it helps with that.  This was a messy
   area to deal with in SSI given the old API; with this change I think
   we could make that part of the SSI code clearer and maybe clean up
   unneeded files in a couple places where cleanup under the old API
   was judged not to be worth the code complexity needed to make it
   happen.
    
   -Kevin
   
   

3. Re: have SLRU truncation use callbacks

   Alvaro Herrera <alvherre@commandprompt.com> — 2011-10-04T17:17:25Z

   Excerpts from Kevin Grittner's message of jue sep 29 14:51:38 -0300 2011:
   > Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
   >  
   > > But I think these changes stand on their own, merely on code
   > > clarity grounds).
   >  
   > After a quick scan, I think it helps with that.  This was a messy
   > area to deal with in SSI given the old API; with this change I think
   > we could make that part of the SSI code clearer and maybe clean up
   > unneeded files in a couple places where cleanup under the old API
   > was judged not to be worth the code complexity needed to make it
   > happen.
   
   Thanks for the review.  I just pushed this.  Note I didn't touch the
   predicate.c stuff at all -- I leave that to you, if you're so inclined :-)
   
   -- 
   Álvaro Herrera <alvherre@commandprompt.com>
   The PostgreSQL Company - Command Prompt, Inc.
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