ZFS on Hardware RAID controller

[freebsd at fongaboo.com]

I was speaking to someone else about this today, and it eventually became 
apparent that we were getting into a sort-of 
Abbott-and-Costello-Who's-on-First confusion... because apparently people 
can mean different things when they use the term 'JBOD'.

What I have always meant when I say 'JBOD' is a (not really) RAID mode 
that simply concatenates the drives into one volume in a serial fashion, 
ie. 'spanning'. Most RAID controllers and RAID-enabled NAS units that I 
have interacted with in my life have offered this mode and referred to it 
as 'JBOD'.

In this kind of mode, the motherboard and the OS still thinks it sees only 
a single volume. So now I am gathering that this is also not ideal for 
ZFS, since it would still not be aware of multiple physical volumes and be 
unable to optimize accordingly.

I'm learning for the first time that sometimes 'JBOD' can also refer to 
each individual drive being mounted separately at least as far as the 
controller and the motherboard is concerned.

I just want to confirm 100% that this is how you are recommending multiple 
drives be configured for ZFS. Because when I started the thread I was 
thinking of JBOD as 'spanning'.


FONG



On Tue, 18 Feb 2014, Eitan Adler wrote:

> On Tue, Feb 18, 2014 at 5:27 PM,  <freebsd at fongaboo.com> wrote:
>>
>> When we spoke, you noted that when installing ZFS on multiple disks
>> connected to a hardware RAID controller, it is best to config it to JBOD.
>
> There are a few reasons for this.
> (a) Hardware RAID serves as a single point of failure: if the
> contoller dies you have neither disk
> (b) As Andrew noted , using hardware RAID means that ZFS won't be able
> to tell which disk is which.  The ZFS management tools won't work as
> expected (they will show only one disk).
> (c) Since ZFS implements RAID itself it can use knowledge about the
> physical disks for better performance
>
> Also see: https://en.wikipedia.org/wiki/ZFS#ZFS_and_hardware_RAID
>
>> I tried to explain this to a colleague, but they were skeptical. Would you
>> (or anyone) be willing to give me a one or two line sales-pitch on
>
> "ZFS does RAID better than the controller."
>
>>  why one
>> should abandon traditional notions of RAID performance in favor of allowing
>> ZFS to do disk management?
>
> The goal isn't to give up on RAID but move its implementation to ZFS.
>
> -- 
> Eitan Adler
> Source, Ports, Doc committer
> Bugmeister, Ports Security teams
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