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Re: SGA rules of thumb?

From: Tim Gorman <>
Date: Tue, 08 May 2007 09:20:30 -0600
Message-ID: <>

RMAN doesn't use the Large/Shared Pool unless you are duplexing (i.e. setting COPIES > 1) your backups. That is, one channel reading from the datafiles writing to more than one channel for writing, which requires some form of inter-process communication which is implemented via "queue tables" in either the Large Pool (preferably) or the Shared Pool when the Large Pool isn't configured.

When not duplexing backups, then there is no need for communication between processes -- the channel process that is reading from the datafiles is the same process writing to backup media, whether tape or disk.

No duplexed backups, no need for IPC queue tables in the Large Pool or the Shared Pool by RMAN. This might explain why no messages or errors?

Don Seiler wrote:
> Alright I'll make this scenario more specific. I'm an RMAN user. My
> large_pool_size has been 16M "forever". The Backup & Recovery
> Advanced Users Guide [1] says that:
> "If LARGE_POOL_SIZE is set, then the database attempts to get memory
> from the large pool. If this value is not large enough, then an error
> is recorded in the alert log, the database does not try to get buffers
> from the shared pool, and asynchronous I/O is not used."
> and that the formula for setting LARGE_POOL_SIZE is as follows:
> LARGE_POOL_SIZE = number_of_allocated_channels *
> (16 MB + ( 4 * size_of_tape_buffer ) )
> Of course I'm backing up to disk, not tape, but it would seem I should
> be using a lot more than 16M. However, I don't see any errors in the
> alert.log with "async" or "sync" in the text, so perhaps the large
> pool is still just fine?
> [1]
> On 5/7/07, Don Seiler <> wrote:
>> I'm wondering if any of you have general "rules of thumb" when it
>> comes to sizing the various pools and db buffer cache within the SGA.
>> I'm going to go back to static SGA rather than risk ASMM thrashing
>> about and causing another ORA-00600 at 2:30 in the morning. I can see
>> where ASMM left the sizes at last, but just wondering what human
>> thinks of things.
>> This is Oracle on RHEL3. sga_max_size is 1456M on 32-bit,
>> going to be (at least) 8192M on 64-bit. The database is a hybrid of
>> OLTP and warehouse. When I say "warehouse", I mean that large
>> partitioned tables holding millions of records exist, and are bulk
>> loaded via external tables and data pump throughout the day. Other
>> than the bulk loading, those tables are read-only.
>> Any advice would be appreciated (yes I've checked the V$*_ADVICE
>> views as well).

Received on Tue May 08 2007 - 10:20:30 CDT

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