Alter Table non-exclusive
Habichtsberg, Reinhard
Hi all
We have 24 x 7 production with our Informix databases. We all heard of Informix Server that ran a year or longer without interruption.
What we wish is the ability to do „Alter tables“ and other administrative tasks that needs „exclusive locks“ without this need. Each change on the table structures which come continuous from our software developement causes trouble with our production: Workflows has to be stopped, dialog programs can’t run.
Is there any chance do these tasks without exclusive locks, perhaps in futur? Or, is there any other way to do these tasks without a production break?
TIA, Reinhard.
Fernando Nunes
First, I think technically, you'll always need some instant of "exclusive access" to the table structures in order to change them. It would not be a good idea to allow two ALTER TABLE statements at the first time. Obviously this should be "serializable" and users should not notice it. But there are other implications which are more tricky, harder to understand and to solve. If you're able to alter a table while it's in use, what would happen to sessions with cursors opened on that table? These cursors could reference a column that is changed or removed.
Note that Informix (specially in newer versions) already has a set of features that try to minimize the impacts of these changes:
- New fragmentation schemes allow for "online" add and drop of fragments
- With a special variable set you're able to "kill" the transactions that would otherwise stop you from adding/dropping a table fragment (many people would like to see this extended to all table ALTER operations)
- You're able to create indexes online
- Many table operations are done "in place" which means that they're very fast (so the "exclusive" period is much shorter than in the past)
In any case, if I put my "Informix DBA" hat, I can easily agree with you and I think that:
- The way "online" index creation works could be extended to other ALTER TABLE operations
- The special variable for ALTER FRAGMENT could be extended to other operations
- Some operations that require "exclusive mode" could possible get done with "shared mode". Sometimes maintaining read access would be "good enough" although not perfect
- The requirements for exclusive in other tables (foreign key/primary key changes) should be perfectly documented and I think they're not
- It should be trivial to identify which sessions are preventing you to run an ALTER TABLE. Many times I spend more time trying to figure out the sessions that must be stopped than actually stoping them and making the change. This increases unnecessarily the time that normal operations are impacted by these changes.
Regarding this last aspect I have created some scripts and defined some techniques that try to ease this procedures. I've written an article about this a few years ago on my blog, and since then I tried to create a script that tells me the sessions "using" a table. It's not nice to say this, but the truth is that i't not as simple as checking "onstat -g opn". Currently I check this and the locks. Lately I haven't been doing this tasks in 24x7 systems, but the persons who do it, usually give me positive feedback on this.
Finally, note that on a 24x7 system, usually people only look at the whole system uptime. I agree with you that this "masks" the fact that some operations may cause partial unavailability of some parts of the data model. That's why I think that whenever possible, some "maintenance" window should be considered, but not on the whole system.
I perfectly understand that a 24x7 system is supposed to be available every time. But the truth is that most of the time you can "turn off" some parts of it at lower peak times without great impact. Web based systems are probably the biggest exception to this idea. Specially if you provide service to different time zones.
Regards.
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Fernando Nunes
Portugal
http://informix-technology.blogspot.com
My email works... but I don't check it frequently...
Art Kagel
Art
Art S. Kagel
Advanced DataTools (www.advancedatatools.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 my employer, Advanced DataTools, 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.
Habichtsberg, Reinhard
If an exclusive lock is required it is impossible to do the job while the table is in use. And some tables in our environment are in use of one or more session permanently. So it doesn’t matter how long the alter job lasts. It cannot be accomplished without a downtime of production.
That is what we (better: my boss) wish(es) to improve.
@Fernando: To find the sessions that use a table I found a tool in the IIUG software corner long ago:
1 #!/usr/bin/ksh
2 # who-access.sh - Find out who is accessing a specified table.
3 #
4 # Author: Jacob Salomon
5 # Date: 07/15/1999
6 # Credits: Original idea from a script by:
7 # Rick Bernstein …
It’s really very usefull and reliable.
Reinhard.
Doug Lawry
continuously by any one, the fix can be to set environment variable
IFX_DIRTY_WAIT (as well as SET LOCK MODE TO WAIT) to the number of
seconds required. This has been available since IDS 9 or 10, but is
underdocumented. Combine this with IDS 11 AUTO_REPREPARE, and prepared
statements on the table will also then be unaffected.
Regards,
Doug Lawry
Fernando Nunes
If the sessions leave open cursors on the tables for a long period than you'll have issues.
Again, having a downtime in production is a "scary way" of putting it. Having to hold or interrupt a few sessions for a few seconds is much more understandable.
As someone else already mentioned, if you're having DIRTY READERS on the tables the situation is much more complex.
And as I reference in my blog article you can use a small trick to prevent new sessions of getting into the way: Open a transaction, grant some privilege to the table and then run the alter inside that transaction. The grant will place a lock on the table's systable record and this prevents other sessions (assuming they're in committed read and lock mode wait) to read the table structure. So the sessions will wait on systables and not on the table itself.
This is useless for dirty readers, but for those you'll have the IFX_DIRTY_WAIT variable.
Meanwhile, while searching interanlly for other things I found a feature request for this. But the issue is complex and not easy to solve... meaning there's no compromise that it will be implemented. It just means that formally IBM knows about this concern.
Regards.
Habichtsberg, Reinhard
Fernando,
we have DIRTY READERS on the tables, yes, we have a „mixed system“ with OLTP and decision support with queries which can last days.
The IFX_DIRTY_WAIT variable may help for short dirty reads. I have to think about it and will do some tests. Meanwhile I thank you for your help.
Reinhard.
Fernando Nunes
In that case, and someone else already explained, IFX_DIRTY_WAIT will certainly help. But, back to basics, you may have a problem if you have queries on the affected tables that last for days. I'm saying this because all this talk comes down to one thing: identify and stop the sessions that prevent the access.
For a complete personal overview about this topic check:
http://informix-technology.blogspot.com/2006/10/when-exclusive-is-not-really-exclusive.html
Regards.
da...@smooth1.co.uk
emmendingen.de> wrote:
> Fernando,
>
> we have DIRTY READERS on the tables, yes, we have a "mixed system" with
> OLTP and decision support with queries which can last days.
>
> The IFX_DIRTY_WAIT variable may help for short dirty reads. I have to
> think about it and will do some tests. Meanwhile I thank you for your
> help.
>
> Reinhard.
>
>
> If an exclusive lock is required it is impossible to do the job while
> the table is in use. And some tables in our environment are in use of
> one or more session permanently. So it doesn't matter how long the alter
> job lasts. It cannot be accomplished without a downtime of production.
>
> That is what we (better: my boss) wish(es) to improve.
>
> @Fernando: To find the sessions that use a table I found a tool in the
> IIUG software corner long ago:
>
> 1 #!/usr/bin/ksh
>
> 2 # who-access.sh - Find out who is accessing a specified table.
>
> 3 #
>
> 4 # Author: Jacob Salomon
>
> 5 # Date: 07/15/1999
>
> 6 # Credits: Original idea from a script by:
>
>
> It's really very usefull and reliable.
>
> Reinhard.
>
> Sent: Wednesday, August 17, 2011 1:26 PM
> To: Habichtsberg, Reinhard
> Cc: informix-l...@iiug.org
> Subject: Re: Alter Table non-exclusive
>
> Some things already only require an exclusive lock momentarily. Most
> alters that add or drop a column or change it's type, adding or dropping
> an index, and others are accomplished in-place or online and do not lock
> that table for long. What are you trying to do that's causing you
> trouble?
>
> Art
>
> Art S. Kagel
> Advanced DataTools (www.advancedatatools.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 my employer, Advanced DataTools, 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.
>
> On Wed, Aug 17, 2011 at 6:17 AM, Habichtsberg, Reinhard
>
>
> Hi all
>
> We have 24 x 7 production with our Informix databases. We all heard of
> Informix Server that ran a year or longer without interruption
> r=0x8001> .
>
> What we wish is the ability to do "Alter tables" and other
> administrative tasks that needs "exclusive locks" without this need.
> Each change on the table structures which come continuous from our
> software developement causes trouble with our production: Workflows has
> to be stopped, dialog programs can't run.
>
> Is there any chance do these tasks without exclusive locks, perhaps in
> futur? Or, is there any other way to do these tasks without a production
> break?
>
> TIA, Reinhard.
>
> _______________________________________________
> Informix-list mailing list
>
> _______________________________________________
> Informix-list mailing list
> Informix-l...@iiug.orghttp://www.iiug.org/mailman/listinfo/informix-list
>
> --
> Fernando Nunes
> Portugal
>
> http://informix-technology.blogspot.com
> My email works... but I don't check it frequently...
Why do have queries that can last days?
Can you either add indexes, denormalize or add most cpus/spindles/
fragments.?
How many threads per query are you using now?
Habichtsberg, Reinhard
Hi Fernando,
I now had the situation to do an „alter table“ on a highly frequented table in production system.
I did the following:
- Waited for dialog free time (got up very early in the morning ;-) )
- Killed some dialog sessions from users, which didn’t log out
- Monitored some sessions, which which where connected to the table in question. I found 6 sessions which were active (you saw current sql statement changing every second). These are workflow batches which run all the time and one 4gl-program that produces certain reports.
- set IFX_DIRTY_WAIT=300
- Ran the following sql commands:
1 set lock mode to wait;
2 begin work;
3 -- lock table table1 in exclusive mode;
4 grant select on table1 to public;
5 grant select on table2 to public;
6 alter table table1
7 add (column1 char(1))
8 ;
9
10 --lock table table2 in exclusive mode;
11 alter table table2
12 add (column1 char(1))
13 ;
14
15 drop view view1;
16
17 create view view1
18 as select
19 table1.column_x,
….
470
471 grant select on view1 to public ;
472 grant select on view2 to public;
473
474 commit work;
I didn’t believe it before but the „alter table“-statements ran successfully. The ALTER TABLE of table1 lasted some minutes. I believe because table1 has 548.675.744 rows fragmented in round robin. I monitored locks with waiters and found, that the 4gl-program waited for systable which was locked by the alter table session.
Unfortunately the 4gl-program crashed with an error:
Date: 01.09.2011 Time: 06:32:51
Program error at "ka_st_sl.4gl", line number 2076.
SQL statement error number -243.
Could not position within a table (informix.table1).
SYSTEM error number -106.
ISAM error: non-exclusive access.
The 4gl-programm ran in modus: lockmode wait. I asked the developer and controlled the code by myself. The time to wait was not restricted.
How can we avoid the crash of a program under these circumstances?
TIA,
Reinhard.
From: Fernando Nunes [mailto:domus...@gmail.com]
Sent: Thursday, August 18, 2011 5:22 PM
To: Habichtsberg, Reinhard
Cc: inform...@iiug.org
Subject: Re: Alter Table non-exclusive
Don't get me wrong. I think every Informix customer has "dirty readers". And currently most of the OLTP systems are also used for decision support queries (or at least for ETL).
In that case, and someone else already explained, IFX_DIRTY_WAIT will certainly help. But, back to basics, you may have a problem if you have queries on the affected tables that last for days. I'm saying this because all this talk comes down to one thing: identify and stop the sessions that prevent the access.
For a complete personal overview about this topic check:
http://informix-technology.blogspot.com/2006/10/when-exclusive-is-not-really-exclusive.html
Regards.
On Thu, Aug 18, 2011 at 3:14 PM, Habichtsberg, Reinhard <RHabic...@arz-emmendingen.de> wrote:
Fernando,
we have DIRTY READERS on the tables, yes, we have a „mixed system“ with OLTP and decision support with queries which can last days.
The IFX_DIRTY_WAIT variable may help for short dirty reads. I have to think about it and will do some tests. Meanwhile I thank you for your help.
Reinhard.
From: Fernando Nunes [mailto:domus...@gmail.com]
Sent: Thursday, August 18, 2011 2:02 PM
To: Habichtsberg, Reinhard
Cc: inform...@iiug.org
Subject: Re: Alter Table non-exclusive
We're having a semantic problem. What you say is true. If an exclusive lock is needed, than you can't get it while the table is in use. But that's a general question regarding locks. That's why we have lock mode...
If the sessions leave open cursors on the tables for a long period than you'll have issues.
Again, having a downtime in production is a "scary way" of putting it. Having to hold or interrupt a few sessions for a few seconds is much more understandable.
As someone else already mentioned, if you're having DIRTY READERS on the tables the situation is much more complex.
And as I reference in my blog article you can use a small trick to prevent new sessions of getting into the way: Open a transaction, grant some privilege to the table and then run the alter inside that transaction. The grant will place a lock on the table's systable record and this prevents other sessions (assuming they're in committed read and lock mode wait) to read the table structure. So the sessions will wait on systables and not on the table itself.
This is useless for dirty readers, but for those you'll have the IFX_DIRTY_WAIT variable.
Meanwhile, while searching interanlly for other things I found a feature request for this. But the issue is complex and not easy to solve... meaning there's no compromise that it will be implemented. It just means that formally IBM knows about this concern.
Regards.
On Thu, Aug 18, 2011 at 6:34 AM, Habichtsberg, Reinhard <RHabic...@arz-emmendingen.de> wrote:
If an exclusive lock is required it is impossible to do the job while the table is in use. And some tables in our environment are in use of one or more session permanently. So it doesn’t matter how long the alter job lasts. It cannot be accomplished without a downtime of production.
That is what we (better: my boss) wish(es) to improve.
@Fernando: To find the sessions that use a table I found a tool in the IIUG software corner long ago:
1 #!/usr/bin/ksh
2 # who-access.sh - Find out who is accessing a specified table.
3 #
4 # Author: Jacob Salomon
5 # Date: 07/15/1999
6 # Credits: Original idea from a script by:
7 # Rick Bernstein …
It’s really very usefull and reliable.
Reinhard.
From: Art Kagel [mailto:art....@gmail.com]
Sent: Wednesday, August 17, 2011 1:26 PM
To: Habichtsberg, Reinhard
Cc: inform...@iiug.org
Subject: Re: Alter Table non-exclusive
Some things already only require an exclusive lock momentarily. Most alters that add or drop a column or change it's type, adding or dropping an index, and others are accomplished in-place or online and do not lock that table for long. What are you trying to do that's causing you trouble?
Art
Art S. Kagel
Advanced DataTools (www.advancedatatools.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 my employer, Advanced DataTools, 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.
On Wed, Aug 17, 2011 at 6:17 AM, Habichtsberg, Reinhard <RHabic...@arz-emmendingen.de> wrote:
Hi all
We have 24 x 7 production with our Informix databases. We all heard of Informix Server that ran a year or longer without interruption.
What we wish is the ability to do „Alter tables“ and other administrative tasks that needs „exclusive locks“ without this need. Each change on the table structures which come continuous from our software developement causes trouble with our production: Workflows has to be stopped, dialog programs can’t run.
Is there any chance do these tasks without exclusive locks, perhaps in futur? Or, is there any other way to do these tasks without a production break?
TIA, Reinhard.
_______________________________________________
Informix-list mailing list
Inform...@iiug.org
http://www.iiug.org/mailman/listinfo/informix-list
_______________________________________________
Informix-list mailing list
Inform...@iiug.org
http://www.iiug.org/mailman/listinfo/informix-list
--
Fernando Nunes
Portugal
http://informix-technology.blogspot.com
My email works... but I don't check it frequently...
_______________________________________________
Informix-list mailing list
Inform...@iiug.org
Fernando Nunes
You managed to do the alter tables easier than you'd usually expect. You saw some user's 4GL programs waiting while you did the ALTER TABLE(s).
That was the good part. The bad part is that one 4GL program instead of waiting exited with and error and you want to understand why and how to avoid it....?
If the above is correct, I'm not certain about what may have caused it.
Some things to collect:
1- The query the 4GL was trying to process
2- Is the query previously prepared?
If my assumptions above are not correct, please clarify.
Regards
Habichtsberg, Reinhard
You got me right.
I now monitored the sql of the running program with onstat –g ses sid –r 1. Under thousands of queries the program produces I found exactely one (by chance) that ran in modus „not wait“. From the code the program should not change the lock mode. It should be always „wait“. But this could be the reason for the crash.
THX, Reinhard.
Fernando Nunes
The LOCK MODE is a session property. Different parts of the code (or even inside stored procedures), this can be changed and not put back.
(By the way, I would love to see some extension to run each query in a particular isolation level and lock mode. I know ANSI doesn't allow it to change - SET TRANSACTION... - but our extension SET ISOLATION... can be used. So imagine a "SELECT .... WITH COMMITTED READ LOCK MODE WAIT 5" )
Besides what you found I don't see any explanation to what happened to you... Maybe others have some clue (although this can explain it...)?
Regarding the change of isolation level and lock mode inside "functions" or procedures, this is also an interesting thing. The fact is that sometimes you want/need to change it and it's not easy to find what is the current one, so that you can reposition it once you're done.
Sometime ago I dig into this and it's possible to have a procedure that implements it... I thought I had an article about this but I don't seem to find it...
Regards
Art Kagel
Art
Art S. Kagel
Advanced DataTools (www.advancedatatools.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 my employer, Advanced DataTools, 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.