Re: [Q] How does Oracle 9i-RAC handle lock to suport HA?

I'm not 100% clear on what you're asking for here, but here's a summary I sent to someone recently:

In the RAC environment, the DLM you refer to is called the global cache service. It keeps track of the instances that are locking blocks. Let's take a simple example of a 3 node cluster with instances 1 and 2 (located on nodes 1 and 2) interested in a block, and the global cache service located on node 3. Instance 1 wants to take out an exclusive lock on a block which has an SCN of 1000. First thing it does is ask the GCS who has that block. If no-one is using it, the GCS tells instance 1 that, and instance 1 then reads the block from disk and communicates the fact that it has taken out an exclusive lock to the GCS. GCS records the fact that instance 1 has the block in exclusive mode, local mode (i.e. in one cache only) with no past images (more on this anon) - status is XL0 for instance 1 (X = exclusive, L = local, 0 = no past image)

Instance 1 now makes a change and commits it (SCN is now 1001). Again it communicates this to the GCS, which updates the SCN information it's tracking. Status is still XL0.

Instance 2 now decides it wants to update another row or even the same row in the same block (Instance 1 has committed so another update to the same row is fine). Instance 2 asks the GCS who has the block. GCS tells instance 2 that instance 1 has it, and tells instance 1 to send it to instance 2 ACROSS THE INTERCONNECT (i.e. no pinging to disk which is where the performance hit came in OPS). Instance 1 either gets it from the data buffer cache if it's still there or from its rollback (or undo) segments, sends the block across the interconnect and then tells the GCS it has it. GCS changes the status for instance 1 to NG1 (N = null - no lock, G = global because the block is now in two caches, 1 = past image - this is no longer the most recent copy of the block). Instance 2 receives the block, tells the GCS it has it, so the GCS now changes the status for instance 2 to XG0 (exclusive lock, global mode, 0 means this is the current image). Instance 2 can now change the block and do whatever it wants.

Now, it's not really that simple, because the GCS CAN'T be on one node only - single point of failure which is what we want to get away from for HA. The GCS is actually duplicated on each node that is running an Oracle instance. That way, each node can tell very quickly where the latest copy of a block is, and can request it from the instance holding that latest copy. Of course explaining it with the GCS on each node gets way complex with arrows pointing every which way when you do it on a whiteboard, so that's why I stick to the simple example when explaining it.

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HTH.  Additions and corrections welcome.

Pete
Author of "Oracle8i: Architecture and Administration Exam Cram"

"Controlling developers is like herding cats."
Kevin Loney, Oracle DBA Handbook

"Oh no, it's not.  It's much harder than that!"
Bruce Pihlamae, long-term Oracle DBA

"Hun Soon Lee" <hunsoon_at_etri.re.kr> wrote in message
news:a1k267$hdu$1_at_news.kreonet.re.kr...


> Hello there.

> I want to know how Oracle9i-RAC handles the lock to suport HA.

> According to the "Oracle9i-RAC concepts",

> it uses DLM(Distribute Lock Manager) to manage the lock.

> I want to know details about how Oracle 9i-RAC handles the lock.

> Does it backup the lock manager to support HA?

> Does it use the lock caching to increase performance?

> etc...

> Thanks for your reading.

> Hun Soon LEE

>

>