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 either. 16G - 64K -> 108mB/s (sequential) There's nothing else using the zpool at the moment, the system is on a separate ssd. 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? Mike
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