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From "Bergquist, Brett" <BBergqu...@canoga.com>
Subject RE: Question on log directory of a derby database
Date Tue, 19 Jul 2011 14:09:33 GMT
Thanks for taking the time to respond Knut.  It is much appreciated.

Some information:

The log files total 64Gb of disk space.  So this clearly went way past the 10Mb of transaction
log.

So the " And the checkpoint will only delete log files older than the oldest transaction that's
still alive."  That is important I think.   So if there was a stuck transaction somehow that
occurred on July 12, for example, then this could in theory keep the transaction logs around
until last night, correct?

Unfortunately the system administrator had already shutdown the database engine before he
called me.  It would not boot the database in a reasonable time.  I was looking at the iostat
and it looked like it was doing about 1Mbs and I used truss to take a look at the system calls
and it was processing transaction log files from July 13'th after quite a while (a couple
of hours of trying to boot the database).  I did a quick calculation and it looked like it
would take somewhere around 17 hours to boot the database.

I looked at the last online backup that the customer had made and again, it had many thousands
of transaction logs in the backup database, so that was not useful either. 

I only had one option.  I knew the system was in a quiet state as there was no applications
accessing the database.  I know this is not recommended but I had no choice but to remove
the old transaction log files and boot the database.  It came up okay and is in operation
okay so I think it will be alright but it could possibly have corruption.  I had to take the
chance however.

I am going to monitor the system and use the syscs_diag.transaction_table to query the transactions
if I see this happen again.  Just a note however, something similar did happen a week ago
and I looks at the transactions and it showed none even though there were thousands of transaction
log files around.  So a question, does the online backup show up as a transaction in the syscs_diag.transaction_table?
 Also, a week ago, there was no locks as reported by the syscs_diag.lock_table (at least the
snapshot of querying this table that I looked at).

Again if there is anything that anyone can think of that I should look at if I see this happen
again, please shout out.

Brett

-----Original Message-----
From: Knut Anders Hatlen [mailto:knut.hatlen@oracle.com] 
Sent: Tuesday, July 19, 2011 9:41 AM
To: derby-dev@db.apache.org
Subject: Re: Question on log directory of a derby database

"Bergquist, Brett" <BBergquist@canoga.com> writes:

> I have a database in production that has been running fine for a few
> years. It started out having about 100K inserts per day into it and
> now is up to about 4.6M inserts per day and this has been working
> fine.
>
> Tonight the customer called because the system was chewing up disk
> space. I had the customer restart the database engine and it is taking
> a long time to boot the database. I had the customer check the "log"
> directory in the database and there were 62K ".dat" files present.
>
> So I am assuming that these are for transactions that have not
> committed, correct?

Yes, but they are not cleaned up until a checkpoint has run (by default
that happens when you have 10MB of transaction log), so they may contain
committed transactions too. And the checkpoint will only delete log
files older than the oldest transaction that's still alive.

> But for the life of me, I cannot figure out what
> transaction could have been in progress and not committed since July
> 12'th. It seems to me this would have exhausted memory or some other
> resource by now.
>
> One other point, an online database backup is done each night by the
> customer. Could this trigger anything like this?

Yes, an ongoing online backup will prevent deletion of log files, since
it needs them to track modifications that happen while it copies the
database.

It could also happen if log archiving has been enabled (using the
SYSCS_BACKUP_DATABASE_AND_ENABLE_LOG_ARCHIVE_MODE procedure). You can
tell whether log archiving is enabled by looking for a line that says

  derby.storage.logArchiveMode=true

in the service.properties file in the database directory.

> Tonight when running
> a utility against the database, the utility failed to acquire locks,
> but there should have been nothing else running but this utility and
> it is single threaded, so there should have been no lock contention.
> It also acts like there is a database backup that is still on going...

I don't think an online backup needs many locks. If you connect to the
database using ij and execute SELECT * FROM SYSCS_DIAG.LOCK_TABLE you
should see which locks are held, which might give some clues.

> Right now, I am just waiting for the database to cleanup and boot so
> that I can get in and examine it. Is there any shortcut or express way
> to to boot the database? Is there any way to monitor the progress of
> this boot cleanup?

I don't know of a way to speed it up. There is a flag that makes debug
builds print more info to derby.log during the recovery phase
(-Dderby.debug.true=LogTrace, I think), but it may be too low-level to
get much useful info in this case.

> Any thoughts or pointers in trying to figure out what is going on will
> be greatly appreciated.
>
> The database in question is Derby 10.5.1
>
> Brett

-- 
Knut Anders



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