Re: Data Mirroring on two data centers -- How to use ASM ?
- From: Tanel PÃder <tanel.poder.003@xxxxxxx>
- To: "Carel-Jan Engel" <cjpengel.dbalert@xxxxxxxxx>
- Date: Sun, 21 May 2006 10:45:10 +0800
Carel-Jan,
Yeah, I picked your email just as the last one in thread, I do definitely agree
with you on that the need for solution should be derived from proper
requirements, not vice versa.
Btw, one more thing if talking about redolog level replication versus storage
replication is that as storage doesn't normally know anything about Oracle's
block formats then it doesn't have any choice but replicate all corruptions to
remote site as well. Oracle's redo apply mechanism would detect most of such
issues...
Tanel.
----- Original Message -----
From: Carel-Jan Engel
To: tanel.poder.003@xxxxxxx
Cc: oracle-l@xxxxxxxxxxxxx
Sent: Sunday, May 21, 2006 3:32 AM
Subject: Re: Data Mirroring on two data centers -- How to use ASM ?
Well, Tanel,
I perfectley understand you read Data Guard between the lines in my answer
;-). However, it wasn't there. I stayed away from mentioning Data Guard
deliberately, because that is just another solution. In this stage, when
requirements are not clear, it's to early to talk solutions.
Having said that, I totally agree with your point. I would add to it that
storage based replication of the whole database also suffers from archiving the
redo. Those blockchanges have to be replicated too. The same happens to
controlfile updates. It will all get worse when you have multiple controlfiles
and online redo log file group members.
Apart from saving bandwidth Data Guard allows you to maintain a delay in
applying archives. This feature helps to survive hardware failures AND 'human'
failures, like accidently dropping/truncating a table, 'new application
features' and so on. Several CT sites have survived serious human errors thanks
to this feature. SAN replication would have replicated the disaster
immediately, leaving the DBA with no other option than a time consuming TSPITR.
For the SAN-based archive replication I'd suggest to set
ARCHIVE_LAG_TARGET=n. This will force log switches every n seconds, also when
the log file isn't full yet.
Best regards,
Carel-Jan Engel
===
If you think education is expensive, try ignorance. (Derek Bok)
===
On Sat, 2006-05-20 at 22:03 +0800, Tanel PÃder wrote:
ï
One more reason to use data guard instead of storage/LVM level replication
in high-activity OLTP environments is that redolog entry based shipping is much
more more fine grained than storage block level replication.
I once asked one EMC admin, they told that the minimum block size for SRDF
is 32k. So if you update one row and commit, you'd need to ship few hundred
bytes to standby, while with SRDF you'd need to transfer 32 kilobytes over the
fibre when the block is written to disk plus you need to continuously transfer
redolog writes before the datafile blocks are sent to remote.
If your management definitely requires SAN based replication, then you
could just keep your archivelogs on a replicated volume/storage and do frequent
log switches to keep the lag small in case of primary failure.
Tanel.
----- Original Message -----
From: Carel-Jan Engel
To: db.mail.1@xxxxxxxxx
Cc: oracle-l@xxxxxxxxxxxxx
Sent: Saturday, May 20, 2006 3:06 AM
Subject: Re: Data Mirroring on two data centers -- How to use ASM ?
Hi Madhu,
I'm wondering your primary 'requirement' of mirroring data across TWO
data centers.
IMHO, mirroring between data centers is a solution, or if you like, tool.
Whatever, it isn't a requirement.
Requirements could be something like:
- After a server failure, the database should be available again within
30 minutes
- After a server failure, no more than 5 minutes worth of transactions
may be lost
- After a database corruption, the database should be available again
within 6 hours
- After a database corruption, no more than 30 minutes of transactions
may be lost
- Restoring of the database to any point in time between now and now - 6
days must be possible
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