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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