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RE: [SPAM] RE: ZFS snapshots

From: Hemant K Chitale <>
Date: Thu, 14 Dec 2006 01:12:34 +0800
Message-Id: <>

Yes, perfect distinction between "restartable" and "recoverable".


At 12:10 AM Thursday, Bobak, Mark wrote:
>First, thanks for the detailed explanation. ZFS sounds pretty
>cool! (Too bad we're moving away from Solaris and towards Linux at
>the moment...)
>Here is where I think we have the disconnect. There is a difference
>between "restartable" and "recoverable".
>When you said "Now, regarding ZFS: A ZFS-snapshot, taken with the
>database online and not in backup mode, is fully
>recoverable. Whatever the state of the datafiles, we have the redo
>logs up to and including the most recent SCN, as well as the
>up-to-date controlfile. The backup itself cannot be opened without
>the redo, but indeed, no online backup can be opened without
>redo. This works."
>That't NOT recoverable. That's restartable, i.e., Oracle does
>instance recovery only, NOT media recovery. This is an important
>point, cause this won't work for backups, only for cloning a database.
>Now, having said that, I think the only miscommunication was the one
>above, regarding terminology.
>At this point, I think I understand (and agree) with what you were saying.
>You can take snapshots of an entire database (data/redo/control), no
>backup, and use that to clone a running database, with no downtime
>on the primary (source) database. This is what I refer to as "restartable".
>You can take snapshots (or any kind of copy, really) of the
>datafiles only, while database is in backup mode, and use this for
>recovery. This is what I refer to as "recoverable".
>One final point I'll make is that a restartable copy is NOT valid
>for recovery. In other words, if you snapshot your primary database
>at time=5, and it's not in backup mode, and it crashes at time=10,
>the best you can do with that snapshot is restore the primary as of
>time=5. There's no way to get from time=5 to time=10. With a
>recoverable copy, you can apply archive logs and roll forward to time=10.

Hemant K Chitale

Received on Wed Dec 13 2006 - 11:12:34 CST

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