Why are we swapping? 2007-05-03 - By herta.vandeneynde@(protected)
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We run Oracle RAC 10.2.0.3 over ASM on a two member cluster. Each member
has 64GB memory and runs x86_64 SLES 10 (2.6.16.27-0.6-smp). We use
EMC's powerpath driver, which doesn't support any higher version yet, so
we're stuck with this version for now.
Hugepages are enabled and used:
$ cat /proc/meminfo
(...)
HugePages_Total: 16384
HugePages_Free: 5425
HugePages_Rsvd: 20
Hugepagesize: 2048 kB
This non-production cluster is used by developers, application and
acceptance testers. Each member runs an Oracle Enterprise Management
Agent, a Big Brother Agent, and 1 instance of each of the 36 databases.
The sum of the sga's of 36 database instances is 21 GB, which are taken
out of the HugePages, and therefore are locked in memory.
The sum of the aggregate pga targets is 3 GB. I know the pga aggregate
target is below the recommended 20%, but these are not end-user
production databases, and the OEM advisor shows current setting to be
adequate: only a few of the development databases show a handful of
executions have been non-optimal executions.
Note that the sum of all SGA_TARGETs + PGA_AGGREGATE_TARGETs is less
than half of available physical memory.
So how come we are swapping? Immediately after reboot, i.e. even before
we have user activity, about half of our swap space is used (4 GB out of
8 GB swap). Following the startup, sar mainly shows swap ins.
FWIIW, a second cluster with the same hardware (apart from faster
storage) and software setup, has 12 databases defined with a total of 31
GB of sga and 7 GB page aggregate target. There's no swapping on the
cluster yet(?) (SwapTotal = SwapFree), but most of the databases are
still empty (data not
imported yet).
This second cluster will become our new production cluster as of next
weekend. Needless to say we're anxious to understand what causes the
swapping on the non-production cluster.
Any suggestions on how to debug this?
Kind regards,
Herta
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