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From "Bergquist, Brett" <BBergqu...@canoga.com>
Subject RE: Can someone explain the use of logged archive backup?
Date Thu, 29 Mar 2012 15:25:01 GMT
Thanks Bryan, but I am a little dense on this one and just want to understand the proper procedure
to use "SYSCS_UTIL.SYSCS_BACKUP_DATABASE_AND_ENABLE_LOG_ARCHIVE_MODE" so that I can either
consider it or not.  

On one system that uses ZFS, we will probably use the SYS_UTIL.SYSCS_FREEZE_DATABASE to freeze,
perform an ZFS snapshot, and SYS_UTIL.SYSCS_FREEZE_DATABASE to unfreeze the database.   I
am concerned of a failure in the code between freezing the database and unfreezing the database
as I believe if the connection is lost, the database will remain frozen with no way to unfreeze.
 I need to test that and maybe patch Derby to perform an unfreeze if the connection is lost.
I have had this happen when I used a script to call IJ to do the freeze, the shell to do the
ZFS snapshot, and IJ again to unfreeze.  The last IJ to unfreeze could not get a connection
and could not unfreeze.  And the database was locked up and could not even be shutdown cleanly.

Another system is an older system that is running UFS so this will not be possible.

Back to "SYSCS_UTIL.SYSCS_BACKUP_DATABASE_AND_ENABLE_LOG_ARCHIVE_MODE".  So the procedure
would be:

	- use this procedure to create a backup of the database and enable log archiving.  Presumably
the database would be created on some other media.
	- if the log directory has not been moved with the from "<database>/log", what does
one do with this?  Periodically copy the file in there to the other media?  How does one determine
the "archive" logs from the "active" logs used for crash recovery or is even necessary.
	- time goes on and another full backup is done.  Does one use SYSCS_UTIL.SYSCS_BACKUP_DATABASE_AND_ENABLE_LOG_ARCHIVE_MODE
again?  I assume that the new archive logs need to be kept with this backup now, right?
	- I guess I confused by the archive logs and where they are located, what they look like,
and what to do with them

Now there is a disk crash that contains the database, a new disk is put in place, formatted,
etc. and is available to the OS.  So the procedure to recover this is to issue a connect like:

	- connect 'jdbc:derby:<database>;rollForwardRecoveryFrom=<path to last full backup
with logs>';

Is that correct?  Is that any different than physically copying the database and all of the
logs to the new disk simply connecting like:

	- connect 'jdbc:derby:<database>'


Any light you can shed on this will be helpful.  I think the Administration guide should go
through this.  I t starts to, but the part about handling the "archive logs" is not really
presented.



-----Original Message-----
From: Bryan Pendleton [mailto:bpendleton.derby@gmail.com] 
Sent: Thursday, March 29, 2012 10:31 AM
To: Derby Discussion
Subject: Re: Can someone explain the use of logged archive backup?

> confused here on the archived logs and the active logs.

In general, there can be multiple logs covering the time between one backup and the next backup,
and those logs must be applied, serially, in the correct order, to recover the database fully.

Once you take that "next" backup, you no longer need the previous backup/logs, though it's
probably wise to establish a reasonable holding period depending on your resources (e.g.,
backup weekly, keep
3 months of backups and logs, destroy the oldest set when you complete the most recent backup,
etc.)

> I need to protect against a media crash and it is not so important to go back to specific
periods of time for the database.

Perhaps you can entirely use lower-level mechanisms, then, such as RAID or other redundant
storage hardware, or a modern filesystem which automatically replicates the underlying data
against the failure of the storage, such as ZFS (http://en.wikipedia.org/wiki/ZFS)

I think it's still wise to have an application-level backup strategy, because sometimes logical
recovery is necessary (e.g., to recover from an application bug or an administrative mistake),
so I think that the exercise you're going through about documenting your backup and recovery
strategies is an excellent one.

And don't forget to test those backup/restore practices, since an untested restore is no better
than no restore at all.

I've found that one useful technique is to provision a secondary machine, which can be MUCH
smaller in terms of CPU, memory, networking, etc., and just has to have enough disk space,
and automate things so that every time I take a backup, my scripts automatically copy the
backup to this spare machine, restore the backup and apply all the logs, and then run a few
queries to satisfy myself that the database is correctly recovered.

thanks,

bryan



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