On Aug 28, 10:37 am, bozon <cur...@crowson1.com> wrote:
> On Aug 28, 12:29 am, "Thomas J. Girsch" <tgir...@NOSPAM.gmail.com>
> wrote:
>
>
>
> > FYI, we've recently encountered a particularly nasty bug that seems to
> > first appear in IDS 10.00.UC6. It involves tables where the record size
> > is larger than the page size. When a session that's accessing a table
> > causes the table to need to add an extent, that session goes into a
> > critical section and stays there for quite some time. When the table is
> > small, it's just for a few seconds, but as the table grows (to around
> > 200 MB, for example) this time increases dramatically. And the problem
> > becomes more acute when a checkpoint hits. Since the checkpoint can't
> > clear until all sessions leave their critical sections, the checkpoint
> > hangs up waiting on the session. In my testing, when my test table got
> > to around 200 MB, we had an 87 second (!) checkpoint.
>
> > To my knowledge, IBM has not yet assigned a bug # to this issue, but we
> > still have a PMR open.
>
> > Downgrading to UC5 eliminated this problem. I also suspect you could
> > work around this by using configurable page size to put the table in
> > pages where a whole record fits on a page, but I have not yet tested
> > this approach. (It took several hours just to get to a point where we
> > realized adding extents was the problem). On systems with 2K pages,
> > tables whose row size is < 2,020 bytes seem to work just fine; it's only
> > when the row size exceeds the page size (and thus get remainder pages)
> > that we start to observe the problem.
>
> > If you're getting unexplained long checkpoints, this might be a place to
> > start.
>
> > This also brings up a side point: I've got the 2,020 number burned into
> > my head as "what fits on a 2K page" from at least the 7.x days, maybe
> > even the 5.x days. Is that still true today? Also, what's the formula
> > for figuring out what fits on a page for larger pages? Do you deduct a
> > flat 28 bytes, or is it some multiple of the page size?
>
> > Thanks,
>
> > - TJG
>
> I believe it is still 28 bytes for larger page sizes. Also, you can
> still only store 255 or 256 records on a page because the number of
> slot entries have not changed.

have should be has in the above.

Also, is this really widespread or is it only with a certain platform?
I was starting to evaluate FC6 but now I am worried because we have
several tables that have more than 2020 max page size.

We found that going to 16k helped performance on larger tables because
you don't have the remainder pages. On our star schema historical
reporting database where every table has large max page sizes we saved
almost 40% because every record wasn't really close to the maximum but
Informix would put each record on its own page if the max size was
greater than 2020.