questions using zfs on raid controllers without jbod option

[Kai Gallasch]

Hi list.

What's the best way to deploy zfs on a server with builtin raid
controller and missing JBOD functionality?

I am currently testing a hp/compaq proliant server with Battery Backed
SmartArray P400 controller (ciss) and 5 sas disks which I use for a
raidz1 pool.

What I did was to create a raid0 array on the controller for each disk,
with raid0 chunksize set to 32K (Those raid0 drives show up as da2-da6
in FreeBSD) and used them for a raidz1 pool.

Following zpool iostat I can see, that there are almost all of the time
no continous writes, but most of the copied data is written in spikes of
write operations. My guess is, that this behaviour is caching related
and that it might be caused by zfs-arc and raid-controller cache not
playing too well together.

questions:

"raid0 drives":

- What's the best chunksize for a single raid0 drive that is used as a
  device for a pool ? (I use 32K)

- Should the write cache on the physical disks that are used as raid0
  drives for zfs be enabled, if the raid controller has a battery
  backup unit ? ( I enabled the disk write cache for all disks)

raid controller cache:

My current settings for the raid controller cache are: "cache 50% reads
and 50% writes"

- Does it make sense to have caching of read- and write-ops enabled
  with this setup? I wonder: Shouldn't it be the job of the zfs arc to
  do the caching?

- Does zfs prefetch make any sense If your raid controller already
  caches read operations?


Cheers,
Kai.