RE: [SPAM] RE: ZFS snapshots
- From: Hemant K Chitale <hkchital@xxxxxxxxxxxxxx>
- To: "Bobak, Mark" <Mark.Bobak@xxxxxxxxxxxxxxx>, "Donahue, Adam" <Adam.Donahue@xxxxxxxxx>, <oracle-l@xxxxxxxxxxxxx>
- Date: Thu, 14 Dec 2006 01:12:34 +0800
Yes, perfect distinction between "restartable" and "recoverable".
Hemant
At 12:10 AM Thursday, Bobak, Mark wrote:
Adam,
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
http://web.singnet.com.sg/~hkchital
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