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.