----- Original Message -----
From: "Karl Denninger" <k...@denninger.net>
Where I am right now is this:
1. I *CANNOT* reproduce the spins on the test machine with Postgres
stopped in any way. Even with multiple ZFS send/recv copies going on
and the load average north of 20 (due to all the geli threads), the
system doesn't stall or produce any notable pauses in throughput. Nor
does the system RAM allocation get driven hard enough to force paging.
This is with NO tuning hacks in /boot/loader.conf. I/O performance is
both stable and solid.
2. WITH Postgres running as a connected hot spare (identical to the
production machine), allocating ~1.5G of shared, wired memory, running
the same synthetic workload in (1) above I am getting SMALL versions of
the misbehavior. However, while system RAM allocation gets driven
pretty hard and reaches down toward 100MB in some instances it doesn't
get driven hard enough to allocate swap. The "burstiness" is very
evident in the iostat figures with spates getting into the single digit
MB/sec range from time to time but it's not enough to drive the system
to a full-on stall.
There's pretty-clearly a bad interaction here between Postgres wiring
memory and the ARC, when the latter is left alone and allowed to do what
it wants. I'm continuing to work on replicating this on the test
machine... just not completely there yet.
Another possibility to consider is how postgres uses the FS. For example
does is request sync IO in ways not present in the system without it
which is causing the FS and possibly underlying disk system to behave
differently.
One other options to test, just to rule it out is what happens if you
use BSD scheduler instead of ULE?
Regards
Steve
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