First, a note about IO modes. IET has two ways of doing IO to whatever actual storage is behind the iSCSItargets it advertises, called 'fileio' and 'blockio' respectively. Fileiomore or less does regular filesystem IO, as if you had opened the backingstorage at user level and were doing read() and write() to it. Blockiodoes low-level direct block IO to the backing storage. All of the followingis specific to blockio.

There are two levels of possible write caching

Many people with ZFS experience have probably heard that ZFS is designedto work safely with disks with write caches, and may also have heardthat this is done with cache flush commands. The details of what getdone when and under what circumstances are a bit complicated, though.

First, I'm only talking about the situation where you give ZFS wholedisks, instead of slices. When you do this, ZFS marks the device asbeing a whole disk in the pool configuration, and the remaining magic isdriven from