very slow disk I/O (nas)

[danbi]

ZFS cache vdevs will help when the system gets some usage -- the cache has to fill. You may check with

# zpool iostat -v

how much of the cache device is full -- it is most efficient when this shows the cache device is almost full (don't worry it will not overflow);

The zlog vdevs can help in read/write cases, because the ZIL will not be allocated/freed on the storage drives and thus will not impact storage performance. You should see immediate effect from the zlog, but not with typical tests like 'copy a file to the filesystem', rather with more multi-tasking load.

There are opinions, that iSCSI in FreeBSD is not yet very well performing. This might or might not be true - and would be the bottleneck in your case. What does

# gstat

show for the I/O utilization of drives while you do your tests?

 

[phoenix]

The zlog vdevs can help in read/write cases, because the ZIL will not be allocated/freed on the storage drives and thus will not impact storage performance.

log devices are only written to, never read from. The only time the data in the log is read is when the system crashes before the data in the log is written to the pool.

The way a log device works is like this:

[data in RAM] ----> [written to log]
     \--later-----> [written to pool]

If a read request is made for data "in the log", it is read from the ARC, not from the log device.