RE: Oracle 11 'cold backup' while DB up.
- From: <rajendra.pande@xxxxxxx>
- To: <mwf@xxxxxxxx>, <howard.latham@xxxxxxxxx>, <oracle-l@xxxxxxxxxxxxx>
- Date: Wed, 8 Jan 2014 12:06:27 -0500
Good point about snapshot. How long is the snapshot duration and if the
databases is essentially fully available during the snapshot
I remember there was a database with a single dedicated app that used
resource manager and did a quiesce ( with I think ALTER SYSTEM QUIESCE
RESTRICTED) but then you have to rely on no long running transactions
and thus coordinate this with all application related processes
So yes the "snapshot" I guess becomes extremely important
Thanks and Regards
- Raj Pande
UBS AG
Platform Services - Operations
Global Service Delivery (GSDM)
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From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Mark W. Farnham
Sent: Wednesday, January 08, 2014 11:49 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
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