ZFS on Hardware RAID controller

[Matthew Seaman]

On 22/02/2014 20:22, freebsd at fongaboo.com wrote:
> If I am planning to use ZFS on a machine where the drive controller
> happens to be one that can be configured for some combination of
> HARDWARE RAID...
> 
> I should configure it so that it does not concatenate/span/combine the
> drives in any way at the hardware level.
> 
> I need to configure it as if it wasn't even a RAID controller, but
> rather in a configuration where the motherboard/filesystem/OS will
> initially see each physical drive as its own volume.

The ideal is to get the RAID controller to pass-through all the actual
disk devices as if it was a host-bus adapter.  Then ZFS can interact
directly with the drives, which generally gives the best results.

Whether you can do this or not depends on what RAID controller hardware
you are using.  3ware controllers used to be able to do this, but since
they were borg'd by LSI a while back I don't know if that is still the
case with their most recent models.

Many LSI RAID controllers -- or controllers using LSI chipsets but
rebadged by companies like Dell or HP -- can be reflashed as host bus
adaptors.  This is a widely recommended setup for building a ZFS box.

Otherwise, if you can't get the drives passed-through, your next best
bet is to create a bunch of single drive RAID-0 devices, and feed those
to ZFS.  mfiutil(8) has a command to do this:

    mfiutil -u0 create jbod 1,2,3,4

This prevents ZFS from having full control over the hardware -- instead,
the RAID controller does various monitoring functions.  ZFS shifts a lot
of the functionality of a dedicated HW RAID card onto the main memory /
CPU of a system, which should both save the cost of a specialised
dedicated card and also allow more flexibility in throwing system
resource at the disk IO subsystem as required by your workload.

The term 'jbod' referred originally to a stand-alone disk array
configured in this way as individual RAID-0 devices hence 'just a box of
disks' but has since been retconned to mean 'just a bunch of disks': any
set of hardrives attached to a machine and presented to the OS as
individual drives or single disk RAID-0 devices, whether in the main
chassis or otherwise.

	Cheers,

	Matthew

-- 
Dr Matthew J Seaman MA, D.Phil.

PGP: http://www.infracaninophile.co.uk/pgpkey
JID: matthew at infracaninophile.co.uk

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