If you know your storage is battery backed-up, can't you just disable sync on the pool?

Dan, I'm new so correct me if I'm wrong..... I understood that synchronous writes are normally acknowledged after being written to RAM and ZIL and if you disable sync it just removes the ZIL (ie sync writes would be acknowledged as soon as written to RAM). In this case BBWC wouldn't help you if power was lost before those were written to disk.

Fredy, Yes, we run on HP SmartArrays with BBWC (see http://www.nexentastor.org/boards/1/topics/7436) but it's not recommended as you lose lots of the reason for running ZFS in the first place.

I might be wrong, but I think what actually needs to be disabled to gain performance in a environment with NVRAM backed up storage device is "syszfsnocacheflush" on Preferences, System; but yes, Sync option should always be enabled otherwise RAM will acknowledge it first. Or I am not sure if when you have write-back enabled on the RAID controller it will tell the OS in any type of write that the data is already safe and you don't need to do anything with "syszfsnocacheflush"

I realize the advantages of running ZFS to manage the disks directly but in this case I am after performance, specially for writing (take advantage of the BBU), which in ZFS could only be achieve with a ZIL device which I am not buying.

Good point.

Our beta Nexenta system runs like that. It gets an iscsi lun from an old EMC box. I believe zfsnocacheflush is needed depending on which system is underneath, for some systems zfs understands that it is battery-backed and will not send a cacheflush command. You can check or just set zfsnocacheflush anyway.

If you do not have a seperate log device (for the ZIL) it will use the pool itself to write the ZIL to (for sync writes).

From what i understand you already answered your own question :)