I have an 8 drive ZFS array (RAIDZ2 - 1 Spare) using 5900rpm 2TB SATA drives 
with an hpt27xx controller under FreeBSD 10 (but I've seen the same issue with 
FreeBSD 9). 

The system has 8gigs and I'm letting FreeBSD auto-size the ARC.

Running iozone (from ports), everything is fine for file sizes up to 8GB, but 
when it runs with a 16GB file the random write performance plummets using 64K 
record sizes.

8G - 64K -> 52mB/s
8G - 128K -> 713mB/s
8G - 256K -> 442mB/s

16G - 64K -> 7mB/s

16G - 128K -> 380mB/s
16G - 256K -> 392mB/s

Also, sequential small block performance doesn't show such a dramatic slowdown 

16G - 64K -> 108mB/s (sequential) 

There's nothing else using the zpool at the moment, the system is on a separate 

I was expecting performance to drop off at 16GB b/c that's well above the 
available ARC but see that dramatic of a drop off and then the sharp 
improvement at 128K and 256K is surprising.

Are there any configuration settings I should be looking at?


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