Roch Bourbonnais - Performance Engineering wrote:
Reported freemem will be lower when running with ZFS than
say UFS. The UFS page cache is considered as freemem. ZFS
will return it's 'cache' only when memory is needed. So you
will operate with lower freemem but won't actually suffer
from this.
It's been wrongly feared that this mode of operation puts us
back to the days of Solaris 2.6 and 7 where we saw a roaller
coaster effect on freemem leading to sub-par application
performance. We actually DO NOT have this problem with ZFS.
The old problem came because the memory reaper could
distinguish between a useful application page and an UFS
cached page. That was bad. ZFS frees up it's cache in a way
that does not cause a problem.
Thanks for the very informative write-up. This clears a few issues for
me, at least.
However, I'm still a bit worried that we'll be running with a lower
freemem value. The issue here is one of provisioning and capacity
planning - or put another way, how do I know when I've got enough memory
if freemem is always low? Having a freemem value we could believe in -
as well as the corresponding performance improvements - was a huge win
for us when Solaris 8 came along, and it makes it very easy to see when
we're out of memory. For example, in our production environments at work
we have automated monitoring which alerts us when freemem drops below a
particular %age of the total physical memory on the machine; it sounds
like ZFS is going to break this.
Is there any way (preferably a simple one) to get the same
easy-to-understand figure when ZFS is in use, or am I missing something?
Thanks,
Phil.
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