IDS Forum

Re: Please vote for a new Informix RFE:

Posted By: Fernando Nunes
Date: Friday, 29 July 2016, at 5:24 a.m.

Art,

I understand the rational behind this request, but I don't think that is
the best way to solve it.
First, we need to distinguish between the logical log buffers (3) and the
hdr logical log buffers (12).
There are situations where we could use "more" of them. For standard
servers the HDR buffers are not an issue. If you see "G" flags in onstat -u
you'll know that we're not flushing the buffers quick enough and it may be
the reason behind this request. On HDR clusters there were situations where
due to network constraints or lack of resources on the secondary the 12
buffers are not enough (in the past there was even a patch provided for a
specific customer if I recall correctly).
Increasing or auto-configuring the HDR buffers would be a good idea.

However, for the "normal" log buffers (3 of them) increasing them is like
hiding the real problem. The problem with log buffers is that we don't
really use them in real/heavy OLTP systems. As I explained in a recent
thread due to the fact that most customers use non-buffered logging we're
constantly flushing the buffers because we do it on every session COMMIT.

What should be done and is already implemented in other RDBMS like DB2 and
Oracle (SQL server seems to lag behind as they recently introduced what we
have) is what is called "GROUP COMMIT".
Basically we don't flush immediately (to allow the buffer to fill up a
little more at least) but we only return on the COMMIT once the info is
actually flushed to disk.
If done properly the applications will not notice any significant delay,
and instead of doing constant flushes that include around one page of
logical log buffer we do less flushes and that allows for the normal
rotation of the three buffers.

Last time I don't think I posted this, so, from a real customer:

IBM Informix Dynamic Server Version 11.50.FC7XH -- On-Line (Prim) -- Up
365 days 13:22:06 -- 18026336 Kbytes

Physical Logging
Buffer bufused bufsize numpages numwrits pages/io
P-1 82 128 10621588 85983 123.53

phybegin physize phypos phyused %used

2:53 1975000 1939119 7499 0.38

Logical Logging
Buffer bufused bufsize numrecs numpages numwrits recs/pages
pages/io
L-1 0 128 175401685 11939562 6501187 14.7 1.8

Subsystem numrecs Log Space used

OLDRSAM 175401555 16032723948

HA 130 5720

As you can see we have 256KB of logical log buffer. But on average we write
to disk only 1.8 pages.
This is caused by the fact they use unbuffered logging (like virtually all
the customers) and because they have a relatively large number of sessions
(should vary between 1000 and 3000). So the rate of COMMITS is very high.

Of course we coudl increase the number of log buffers... but that's not how
this problem was solved in other technologies and there are probably good
reasons for that.

Finally, there is also an RFE to implement GROUP COMMITS:

https://www.ibm.com/developerworks/rfe/execute?use_case=viewRfe&CR_ID=45166

Feel free to vote ;)

On Thu, Jul 28, 2016 at 7:31 PM, Art Kagel <art.kagel@gmail.com> wrote:

> Folks:
>
> I have entered a new RFE # 92334. It requests that IBM make the number of
> logical log buffers configurable, dynamically adjustable, and include an
> AUTO_ autonomic tuning capability for this resource:
>
> https://www.ibm.com/developerworks/rfe/execute?use_case=viewRfe&CR_ID=92334
>
> Please vote for this if you think it might be useful to you.
>
> Art
>
> Art S. Kagel, President and Principal Consultant
> ASK Database Management
> www.askdbmgt.com
>
> Blog: http://informix-myview.blogspot.com/
>
> Disclaimer: Please keep in mind that my own opinions are my own opinions
> and do not reflect on the IIUG, nor any other organization with which I am
> associated either explicitly, implicitly, or by inference. Neither do
> those opinions reflect those of other individuals affiliated with any
> entity with which I am affiliated nor those of the entities themselves.
>
>
> _______________________________________________
> Informix-list mailing list
> Informix-list@iiug.org
> http://members.iiug.org/mailman/listinfo/informix-list
>
>

--
Fernando Nunes
Portugal

http://informix-technology.blogspot.com
My email works... but I don't check it frequently...

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