ZFS for raid

Matthew Seaman matthew at FreeBSD.org
Tue Jan 15 22:49:15 UTC 2019


On 15/01/2019 21:48, Paul Schmehl wrote:
> If you're going to setup drives with zfs for raid, do you need a
> hardware controller? My impression is that you don't, but I want to know
> for sure before I order a server.

ZFS provides all the functionality you'ld otherwise get from a RAID
controller.  What you need hardware-wize depends on how many drives you
need to manage.

If it's just a few then you might be able to use the SATA headers on
your motherboard.

If it's many drives then what you need is a Host Bus Adapter (or
several) -- if you're undecided about ZFS and want the option of
switching to a RAID card, there are some RAID cards can be reflashed as
HBAs and vice-versa.  Although generally the experience with ZFS is that
it beats out HW RAID pretty much across the board.

Some RAID controllers have a pass-through mode which effectively
presents the individual drives to the OS.  This isn't a bad choice.

If you're stuck with a RAID controller with no possibility of reflashing
or pass-through mode, but you really want ZFS, then it is possible to
make each drive into a single-disk RAID array and manage those via ZFS,
but this is non-ideal.  ZFS works best when it has maximum access to
each of the individual disks, and single-disk RAIDs interfere with that.

What you certainly do need with ZFS is plenty of RAM, as ZFS makes
extensive use of RAM for caching or bufferring disk IO.  Somewhat
counter-intuitively though best performance results are often obtained
by limiting the amount of your RAM that ZFS can monopolise, since that
leaves more memory available to your application software.

> If there's no raid controller, can you setup raid using zfs during the
> OS install? If you boot from the cd/dvd, do you get the option to
> configure raid using zfs?

Sure.  The installer lets you allocate disks to your zpool, and create
various types of mirroring, striping, raidz, raidz2, etc.  ZFS is pretty
forgiving, and it is possible to do quite radical surgery directly on
the zpool you're booted from without needing to reboot the OS.

	Cheers,

	Matthew

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