Is a SUSPEND really necessary with EMC SnapView
- From: Hemant K Chitale <hkchital@xxxxxxxxxxxxxx>
- To: ORACLE-L@xxxxxxxxxxxxx
- Date: Sun, 22 Aug 2004 22:13:23 +0800
There has earlier been discussion [with me asking questions about
SnapShot/SnapCopy implementations
and later also responding to questions] about how an Oracle Hot Backup is
done with
SnapShot/SnapClone mechanisms.
In my organisation I do have a few SnapClone implementations on Hitachi and
EMC SANs.
I use BEGIN BACKUP and END BACKUP before and after the split but do not use
aSUSPEND.
Recently a colleague of mine tested an EMC SnapView SnapClone of a
productiondatabase
using the steps
on primary
BEGIN BACKUP
split
END BACKUP
on secondary
STARTUP MOUNT {OPEN fails with Recovery Required, as expected}
RECOVER DATABASE
OPEN
Run "dbv" on all datafiles
However, later, when we started querying the clone data we found corrupt
indexes.
ANALYZE TABLE VALIDATE STRUCTURE CASCADE failed for a few tables.
That is when I came in to the picture. I found an EMC doc on 8i [and also
another doc on 9i the
EMC engineer sent me] specifically state why a SUSPEND is required. Both
EMCengineers
at my site categorically stated that they use BEGIN BACKUP and END BACKUP
butnot a SUSPEND
at other sites. Yet the EMC docs state that a SUSPEND is required.
How have your experiences been ?
{as for the "corrupt database" I have asked the DBA, SysAdmin and EMC
engineers to schedule
another test, still without the SUSPEND as the EMC engineers swear that it
isnot required}.
http://www.emc.com/pdf/partnersalliances/oracle/clarFC4700_snapview_oracle8i.
'pdf[1]
Page 16
"The use of ALTER SYSTEM SUSPEND is often questioned in backup scenarios
where use of different
SNAP or mirror-splitting technologies is leveraged to perform
?instantaneous?, or very rapid, data
duplication.
With hot backups, the physical data content of the various Oracle files
continue to change even after a
tablespace has been placed into hot backup mode.
Oracle relies on the ordering sequence of how various OS writes to the files
are organized to ensure that the
logical content relationship of the files on durable media allow a correct
recovery to be performed in the
event of unexpected server or storage system failures.
When the Oracle files are distributed over a number of system disk devices,
acommon practice in most
Oracle deployments to minimize the impact of single device failure, and to
improve general I/O
performance, the different ?devices? have to be duplicated together.
However, when we are starting the SnapView sessions on the different
devices,they are not started
atomically. Timing windows may exist as a result. The set of Oracle files
being snapped may appear to
have lost the required I/O order sequencing.
The ALTER SYSTEM SUSPEND command suspends physical I/Os to the various
Oracledatabase files
until ALTER SYSTEM RESUME is executed. With I/O suspended to the various
database files, there will
be a temporary quiescence of OS level I/O to the various Oracle files.
Duringthis window, the physical
content of all the Oracle files would be content-consistent. When all the
required SNAP sessions are
successfully started within this window, everything should then be working
correctly."
Hemant K Chitale
Oracle 9i Database Administrator Certified Professional
http://web.singnet.com.sg/~hkchital
[2]
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--- Links ---
1
http://www.emc.com/pdf/partnersalliances/oracle/clarFC4700_snapview_oracle8i.pdf
2 http://web.singnet.com.sg/~hkchital
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