how big is the SGA in oracle (well more specifically the buffercache ) ?

when you start up oracle, the pages are not all touched. so if you start
do a scan then all those pages are touched in the SGA (shmem segment)
and will show up as memory in use. otherwise they wouldnt. yet if not
used. that could explain why. so cached in that case is shmem - not os
disk or filesystem buffers


CLEMENS.BLEILE@(protected):
> Hi Mihail,
>
> asking this question an ASMLIB-developer I got this reply:
>
> "ASMLib uses the block devices directly, and does no caching.
> It's direct I/O only. There can be many other things caching, of
> course."
>
> I actually made a quick test:
>
> 1. created a table with 350MB size.
> 2. My machine has 1GB RAM
> 3. After the first full table scan my cache went up to 540MB
> 4. alter system flush buffer_cache;
> 5. Repeating the full table scan showed that the same amount of physical IO happened (iostat -x) as the first time
> 6. I verified this on the storage (I have an iSCSI-target on Linux), because I see IO there as well.
>
> I.e. subsequent full-table-scans do not take data out of the cache, i.e. direct IO happens. The mysterious thing is: What's in the cache?
>
> Ciao
>
> Clemens
>
> --- Originalnachricht ---
>  
>> Hi all,
>> I observed a strange (to my eyes) behaviour in my SLES9 + ASM
>> + ASMLib +
>> Oracle10.2 RAC setup.
>> After upgrading my 2 nodes to 7GB RAM each I started the RAC
>> instances
>> and used top to observer the memory and CPU usage.
>>
>> Then I launched a SELECT statement which accesses lots of data
>> and
>> performs group by and order by operations. What seems strange
>> to me is
>> that
>> 'cached' grew from 2GB to about 5GB while the statements
>> finishes.
>>
>> So the question is does Oracle ASM use my devices as block
>> devices and
>> does it hold the blocks in some cache?
>>
>> I am using partitions from shared scsi disk array which are
>> marked as
>> ASMLib DISKS. The OS is 32bit SLES9 SP3 + updates (kernel
>> bigsmp-276).
>>
>> Here is the output of cat /proc/meminfo :
>> MemTotal:    7267640 kB
>> MemFree:     250172 kB
>> Buffers:     117076 kB
>> Cached:     5180112 kB
>> SwapCached:    28880 kB
>> Active:     6236304 kB
>> Inactive:     375548 kB
>> HighTotal:   6422320 kB
>> HighFree:      2108 kB
>> LowTotal:     845320 kB
>> LowFree:     248064 kB
>> SwapTotal:   8401976 kB
>> SwapFree:    8292396 kB
>> Dirty:        1616 kB
>> Writeback:       0 kB
>> Mapped:     4014284 kB
>> Slab:       133152 kB
>> Committed_AS: 6998544 kB
>> PageTables:   240464 kB
>> VmallocTotal:  112632 kB
>> VmallocUsed:   12060 kB
>> VmallocChunk:  100284 kB
>> HugePages_Total:   0
>> HugePages_Free:    0
>> Hugepagesize:   2048 kB
>>
>> 10x in advance,
>>
>> Mihail Daskalov,
>> Oracle DBA    
>> www.technologica.com
>>
>>
>>
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>
>
>  


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