IDS Forum

Re: Forerground writes?

Posted By: JACQUES RENAUT
Date: Monday, 13 September 2010, at 12:07 p.m.

In Response To: Forerground writes? (MIKE MAGIE)

Mike wrote:
11.50.UC6W2X4
Solaris 10

All -

Since our upgrade to version 11 (from 10) we are now seeing foreground writes showing up in our onstat -F output. Not a huge amount but enough to make me say hmmmm. Admittedly, we do have a poorly configured physical log size right now - onstat -g ckp suggests that it be increased from the 2 gig it is now to ~50 gig - but excluding that from the scene (if possible or appropriate) what would cause these new foregroung writes - assuming our processing has remained the same since v10? We have cleaners maxed out at 128, 750,000 buffers, 128 lrus, and the max/min is back to 2 and 1, (when I increased this to 60 and 50 per the doc. re. new ckpt methods I saw thousands of FG writes. The instance supports a great deal of drop big table - create big table - load big table - index big table, but we ran these jobs with the current config under v10 w/o seeeing ang FG writes - in fact the buffer pool was smaller.

I will say that I am not 100% or even 32% confident in our underlying I/O system (giant shared SAN) nor the CPU setup on this server (Suns Virtualized thingee where we have 128 virtual cpu's via cores or whatever). All this said I am more interested in the mechanics behind why the fg's suddenly started occuring with v11 - thinking it has something to do with the new non blocking ckpts. if the answer is 'your physical log is too small' then cool, we will have the eunuchs admins carve us out bigger luns.

MM

Response:
Well, most people think of foreground writes as when the engine tries to get a free buffer for a page, but there are no non-dirty buffers so 1 has to be written before a free non-dirty buffer is available. However, if you have lru min/max set to 2 and 1 then there should be pretty much 0% chance this is the type of foreground write you are seeing. I then was looking into the code to see when the foreground write profile counter is increased and compared the two versions. It appears as though nothing has changed in that sense. But 1 of the ways we can increase the foreground write counter is when doing the btree merge. If during a bttree merge a checkpoint pops up before the merge pops out of it's critical section it looks for certain dirty pages that are part of the merge and it forces those buffers to be flushed, prior to allowing the checkpoint to happen, and it counts those pages as foreground writes. Of the places we do increase the foreground write counter, that seems the most likely place of the 4 or 5 places it gets bumped.

Jacques Renaut
IBM Informix advanced support
APD Team

Messages In This Thread

• Forerground writes?
  MIKE MAGIE -- Monday, 13 September 2010, at 11:40 a.m.
  • Re: Forerground writes?
    JACQUES RENAUT -- Monday, 13 September 2010, at 12:07 p.m.
    • Re: Forerground writes?
      DAVE GRIFFEN -- Monday, 13 September 2010, at 5:03 p.m.
  • Re: Forerground writes?
    Lester Knutsen -- Monday, 13 September 2010, at 12:16 p.m.
  • Re: Forerground writes?
    MIKE MAGIE -- Monday, 13 September 2010, at 12:34 p.m.
  • Re: Forerground writes?
    Art Kagel -- Monday, 13 September 2010, at 1:07 p.m.
  • Re: Forerground writes?
    DAVE GRIFFEN -- Monday, 13 September 2010, at 5:48 p.m.

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