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MQSeries.net Forum Index » General IBM MQ Support » Circular logging and media recovery ?

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mpuetz
PostPosted: Fri Feb 08, 2002 7:45 am    Post subject: Reply with quote

Centurion

Joined: 05 Jul 2001
Posts: 149
Location: IBM/Central WebSphere Services

Hi,

I have been chewing on this for quite a while now:
The manuals state that circular logging isn't sufficient
for media recovery, i.e. if your /var/log/qmgrs platter
falls of the table you cannot recover persistent messages.
Or can you, in principle, if the oldest message is still
in the circular logs ? I.e. if you have plenty of log space
and you *know* the maximum lifetime of all your messages
can you rely on circular logging for recovery ?



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kolban
PostPosted: Fri Feb 08, 2002 10:55 am    Post subject: Reply with quote

Grand Master

Joined: 22 May 2001
Posts: 1072
Location: Fort Worth, TX, USA

Unless your hard drives become corrupted or lost, using circlular logging your messages can be restored to a consistent, transactionally complete state. Linear logging ONLY buys you media recovery.

In my opinion, if your data can not afford to be lost in the event of a hardware failure, you should be using disk mirroring hardware technologies and not involve yourself with a logical, software burden. In this case, circular logging is more than sufficient.
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ChrisW
PostPosted: Sat Feb 09, 2002 1:37 am    Post subject: Reply with quote

Voyager

Joined: 20 May 2001
Posts: 78
Location: UK

One problem with circular logging is that should say, the log become corrupted (which I have seen happen) then the only options you have are either to revert to a full disk backup (taken when the Queue Manager is down) or rebuild the Queue Manager from scratch. The log and the queue files must always be in step.

Restoring a backup may result in duplicate mesages being sent in addition to any that were lost. Rebuilding the Queue Manager may result in lost messages.

With that in mind, in my opinion applications that use MQSeries should be designed to be able to re-send messages if required, and to cope with duplicates.
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EddieA
PostPosted: Sun Feb 10, 2002 10:04 pm    Post subject: Reply with quote

Jedi

Joined: 28 Jun 2001
Posts: 2453
Location: Los Angeles

Chris,

If the Circular log files are corrupt, provided this doesn't happen when you have a message 'in-doubt', you can recover from this by copying the 'Active' directory and the amqhlctl.lfh file from a newly created Queue manager to replace the 'broken' one.

Cheers,

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jhalstead
PostPosted: Fri Feb 15, 2002 11:03 am    Post subject: Reply with quote

Master

Joined: 16 Aug 2001
Posts: 258
Location: London

I'm right in the middle of this myself.

Just to clarify my understanding w.r.t. circular logging - where all transactions considered will be simple puts followed by s commit, or gets followed by commit.

The file systems are on mirrored disks so a single disk blow out shouldn't cause any serious trouble.

Now it appears to me that the risk to the queue manager is now if either the queue or log files get corrupted... (Out of interest how does this happen - does MQSeries just go mental?) or if somebody deletes them (idiot or disgruntled employee).

If a queue is damaged the best we can do is replace with an old backup of the queue file (perhaps from when the queue manager was initially built)
However we're going to lose all messages which existed on the queue. Is this correct?

If the logs are corrupt we can do as suggested in an earlier point and replace with an old set of logs + control file, probably from when the queue manager was built). The messages are still on the queue files so there should be no problem?

Is this a reasonable understanding? What is the probability of the the queue or log files corrupting?

Many Thanks!

Jamie
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ChrisW
PostPosted: Mon Feb 18, 2002 3:42 am    Post subject: Reply with quote

Voyager

Joined: 20 May 2001
Posts: 78
Location: UK

One time the logfiles were corrupted was when a UNIX Administrator was looking to find some more disk space and decided that compressing the MQSeries log was the thing to do! When the logs were uncompressed the Queue Manager could no longer read them.

I have also seen a corruption on OS/390. The root cause of this was IPLing an lpar not running MQ but some definition (I'm not an OS/390 Sysprog) was still pointing to the lpar runnning MQ. Lots of strange thing happened including the log getting corrupted. Conditional restart would not work either (under 1.3 I think, maybe 2.1) so the only solution was to rebuild the Queue Manager.

Both of these situations were as a result of human error so I guess that is the most likely cause.
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