RE: Oracle 11 'cold backup' while DB up.
- From: "Powell, Mark" <mark.powell2@xxxxxx>
- To: 'ORACLE-L' <oracle-l@xxxxxxxxxxxxx>
- Date: Wed, 8 Jan 2014 17:34:28 +0000
I believe Oracle has the suspend and resume commands of alter database for use
in schemes like this.
________________________________
From: oracle-l-bounce@xxxxxxxxxxxxx [oracle-l-bounce@xxxxxxxxxxxxx] on behalf
of Mark W. Farnham [mwf@xxxxxxxx]
Sent: Wednesday, January 08, 2014 11:48 AM
To: howard.latham@xxxxxxxxx; 'ORACLE-L'
Subject: RE: Oracle 11 'cold backup' while DB up.
I guess it depends on what they mean by a snapshot, which might under the
covers essentially be a broken mirror. Even for that, though, you need a period
long enough in “hot backup mode” to get the full block redo to cover any Oracle
level blocks that might have been fractured as writes of the underlying
physical sectors at the OS level. “Long enough” can be zero time if the storage
manager has a way to suspend writes and effectively tell all the dbwr to wait
while the snap is made. Some volume managers for a time (maybe some still do)
had a “quiesce” command that more or less paused new writes. That could work,
but it seems fragile to me and onerous to prove that dbwr cannot somehow write
a partial Oracle block in that model.
If the underlying physical sector size matches the oracle block size (which is
more plausible now with physical sectors of 4096 available after a long history
of having essentially everything being 512), I suppose you could eliminate the
possibility of fractured block writes. You’d still have a tough job proving
that was correct in all possible scenarios.
The other way is to read the database blocks through Oracle’s read model, which
is what RMAN does, or turn on “hot backup mode” so that sufficient to recreate
changed blocks is in the redo stream. I’ve trusted “hot backup mode” since
6.0.37.x, and I trust Oracle to get their own read model correct for RMAN, but
I don’t think I trust a third party to keep up with possible changes.
I suppose if you knew enough about the internals of ASM and this product only
works with ASM there might be a way to eliminate the possibility of fractured
writes. Again you’d have the problem of staying lockstep current and reproving
reliability every time Oracle changed.
Still, folks are very clever and this could be a freshly born chick. It would
be interesting to hear the product name and the claimed technical
specifications of how it works.
Delphix, for example, accomplishes the goal that seems to be wanted, but I
would not describe it as a cold backup. (Look to Delphix for their own
description of their product.)
mwf
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On
Behalf Of Howard Latham
Sent: Wednesday, January 08, 2014 6:57 AM
To: ORACLE-L
Subject: Oracle 11 'cold backup' while DB up.
Ive just been told there's a tool that will do a 'cold backup' of a live
database by taking a snapshot of the db at a particular time.
The only way I have heard of achieving this is by having mirroring then
temporarily Breaking the mirror and backing up the mirror . Any good opinions
or Ideas on this would be welcome.
--
Howard A. Latham
--
//www.freelists.org/webpage/oracle-l
Other related posts:
