On Wed, 6 May 2009, Troy Nancarrow (MEL) wrote:
Please forgive me if my searching-fu has failed me in this case, but
I've been unable to find any information on how people are going about
monitoring and alerting regarding memory usage on Solaris hosts using
ZFS.
The problem is not that the ZFS ARC is using up the memory, but that the
script Nagios is using to check memory usage simply sees, say 96% RAM
used, and alerts.
Memory is meant to be used. 96% RAM use is good since it represents
an effective use of your investment.
In the old days there was concern if an application or application's
data could fit in RAM, or that if there was a shortage of RAM, then
there could be a lot of activity on the swap device and the system
would slow to a crawl.
Nowadays RAM in server applications is primarily used as cache. It
could be caching application executable pages, shared library
executable pages, memory mapped file pages, and normal filesystem
data. Caching is good so if a busy server shows a lot of memory free,
then perhaps someone wasted money purchasing more RAM than was needed.
Or perhaps this extra memory is in reserve for that unusual high load
day.
If there is insufficient RAM, there may still be 96% RAM in use, but
since there is less useful caching, other metrics such as hard paging
rates or disk access rates may go up, leading to a slow system.
It seems like this Nagios script is not very useful since the notion
of "free memory" has become antiquated.
Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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