On 05/17/12 15:03, Bob Friesenhahn wrote:
On Thu, 17 May 2012, Paul Kraus wrote:
Why are you trying to tune the ARC as _low_ as possible? In my
experience the ARC gives up memory readily for other uses. The only
place I _had_ to tune the ARC in production was a couple systems
running an app that checks for free memory _before_ trying to allocate
it. If the ARC has all but 1 GB in use, the app (which is looking for
On my system I adjusted the ARC down due to running user-space
applications with very bursty short-term large memory usage. Reducing
the ARC assured that there would be no contention between zfs ARC and
the applications.
If the system is running one app which expects to do lots of application
level caching (and in theory, the app should be able to work out what's
worth caching and what isn't better than any filesystem underneath it
can guess), then you should be planning your memory usage accordingly.
For example, a database server, you probably want to allocate much of
the system's memory to
the database cache (in the case of Oracle, the SGA), leaving enough for
a smaller ZFS arc and the memory required by the OS and app. Depends on
the system and database size, but something like 50% SGA, 25% ZFS ARC,
25% for everything else might be an example, with the SGA
disproportionally bigger on larger systems with larger databases.
On my desktop system (supposed to be 8GB RAM, but currently 6GB due to a
dead DIMM), I have knocked the ARC down to 1GB. I used to find the ARC
wouldn't shrink in size until system had got to the point of crawling
along showing anon page-ins, and some app (usually firefox or
thunderbird) had already become too difficult to use. I must admit I did
this a long time ago, and ZFS's shrinking of the ARC may be more
proactive now than it was back then, but I don't notice any ZFS
performance issues with the ARC restricted to 1GB on a desktop system.
It may have increase scrub times, but that happens when I'm in bed, so I
don't care.
--
Andrew
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