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ZFS snapshots

From: Donahue, Adam <>
Date: Sat, 16 Dec 2006 21:22:06 -0500
Message-ID: <>

I am just sending this FTR. It was in follow up to our discussion of ZFS snapshots and online backups, which I believe intermingled two related but separate issues. I've outlined them below.

I. ZFS Snapshot-Based Refreshes

The goal is to find a fast way to periodically update a pre-prod environment using a copy of the current production database, and to do so with minimal production outage and without requiring a long restore, recover operation.

So we have a production environment, running on ZFS, from which we take a base snapshot and populate the target server. (This is effectively an entire copy of the filesystem, and will thus take some time, but will only need to be performed once.) Note that the copy of the snapshot is never opened by Oracle - it's merely a backup of the production database as of a point in time.

Going forward, we periodically snapshot production (without needing to bring it down) and applying that snapshot incrementally to the most recent target snapshot copy. If we do this frequently enough we end up with a series of small updates to the copy.

And all of this, for our use case, is to simplify the occasional rebuilding of the pre-production database. To do the rebuild, we simply clone the latest ZFS snapshot on the target, freeing us from the need to restore (duplicate) from RMAN, and allowing us to avoid any production down time. It's just a clone of the most recent snapshot on the target.

But, of course, that clone is inconsistent, in that the snapshot was taken when the datafiles may have been in an inconsistent state (SCN wise). Because we took the datafile snapshot first, though, we can recover them using the snapshotted logs, allowing us to open the database. Voila!

(In case it's not clear, pre-production will diverge from production at times, hence the need for the refresh. This is an operational requirement here.)

I haven't yet heard an argument as to why this wouldn't work, and I'm fairly convinced, indeed, that it will.

II. BEGIN BACKUP ... END BACKUP AND PITR I posited that a hot backup via BEGIN BACKUP ... END BACKUP would not allow one to recover the associated datafile(s) to a point in time between those calls.

I found this note in the Oracle documentation that led me to this hypothesis (emphasis mine):

	Like any other backup, an online backup is composed of image
	copies of all the datafiles that make up a tablespace. The point
	remember is that as these files are being backed up they may
also be
	in the process of being written to by the detached process DBWR.
	characteristics of an online backup are
	      o  users are allowed normal access to all online
	         thus, users can access the tablespace being backed up.
	      o  when used for recovery the backup can only be used to
	         to the most recent state of the database, not to a
	      o  only the database files comprising a tablespace are
	         up; the log files are being archived and the control
	         does not need to be backed up if there had been no
	         structural change to the database since the last
control file

Yet if every change during the hot backup copies the changed block to the log, I'm not exactly clear why PITR wouldn't be possible. It's a lot more data in the log, sure, but the implication is that every SCN reflected in the redo stream is associated with the block that changed, indeed, the entire block change.

So perhaps I am misinterpreting that statement.

[Note that the statement The point to remember is that as these files are being backed up they may also be in the process of being written to by the detached process DBWR does not apply to the ZFS scenario I outlined above, hence my conclusion we could do without ever having the bring the datafiles into backup mode.]


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Received on Sat Dec 16 2006 - 20:22:06 CST

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