Setting up gmirror

[Jeremy Chadwick]

On Wed, Oct 01, 2008 at 11:19:01AM +0200, Wojciech Puchar wrote:
>> And what exactly do you classify controllers such as the Promise TX4310
>> and the Promise S150 SX4 as?  The TX4310 could be classified as
>> "software RAID", but a few of the features are offloaded onto the
>> controller.  The SX4 is the same way, but has actual on-board cache.
>
> si it do something by hardware. anyway - i don't think it will actually  
> make it faster under FreeBSD (contrary to windoze).
>
> it's simply not worth money.
>
>> You like to declare everything as "software RAIDs", while I like to
>> discern the difference between them using (what I believe to be) more
>> accurate terminology:
>>
>> BIOS-level RAID (Adaptec HostRAID, Intel MatrixRAID; "chipset" RAID)
>> OS-based RAID (gvinum, ccd, etc.)
>
> what are the difference between 1 and 2?
>
> there are none.

With regards to Intel MatrixRAID, you're correct -- all the feature does
is provide disk pairing features via the southbridge, and provide an
option ROM which configures and manages metadata on the disks for the OS
to read and make use of.  All the I/O operations still have to go
through the CPU, and nothing is off-loaded.

The reason I discern the difference is because if you tell a user "yes,
your Intel MatrixRAID is software RAID", they become confused, since the
feature is provided on a dedicated chip (ICHx).  "But software RAID is
done in the operating system, like gvinum!"

There is also one difference which you're forgetting: booting.

FreeBSD has a long-standing history of not being able to boot off of
most anything other than UFS and gmirror; I believe gvinum might work,
but I've only been able to find confirmation that gmirror does.  I'm
under the impression gstripe doesn't, and ZFS definitely does not.

By using an additional (lower) layer that the OS has no control over
(e.g. MatrixRAID), you can get around this limitation, because the OS
considers the disks a single device (e.g. /dev/ar0).

Of course, you run into other problems using MatrixRAID on FreeBSD,
specifically when a disk fails (see my Wiki page), and the negatives
there easily outweigh the positives.

If you have a RAID-0 configuration you want to boot from, I've no idea
what will work.

If you have a RAID-1 configuration you want to boot from, gmirror is a
good choice.

If you have a RAID-5 or other configuration you want to boot from, a
software or hardware RAID controller would be sufficient (see above).

-- 
| Jeremy Chadwick                                jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |