Please help - adaptec 2820sa not ... RESOLVED

Eric Anderson anderson at
Mon Jun 19 03:43:42 UTC 2006

Ensel Sharon wrote:
> On Sun, 18 Jun 2006, Eric Anderson wrote:
>>> 6.1 sysinstall does in fact see both 2820sa controllers, and when I put in
>>> a single 160GB sata drive, it does see that single drive and I can install
>>> onto it, etc.
>>> Sysinstall does _not_ see my 2.7TB raid6 array.  I suspect that if it were
>>> smaller than 2TB, it would see it correctly.
>>> I have a number of options with which to deal with this, all of which
>>> involve either wasting money or wasting disk space.  Fantastic.
>> Right - FreeBSD doesn't recognize >2TB LUNs.  You should make two LUNs, 
>> and concat them or stripe them with GEOM.  Don't use the large partition 
>> for the OS either.
>> You shouldn't waste either disk space or money.
> Let's say I have 8 disks.
> Let's say I require raid6.
> If I make one array, I lose 25% to raid overhead.
> If I make two arrays, I lose 50% to raid overhead.
> So it would seem that my inability to use a >2TB LUN does indeed lose me
> both space and waste money.

I suppose if you call increased redundancy 'waste' then yes.  Wouldn't 
two 4 disk RAID 5 arrays give you similar (not exactly, but close) 
redundancy to the RAID6 option, and keep your space up?  If you require 
RAID6, then the point is mute, and you're stuck with multiple RAID6 
arrays unless your controller allows you to carve LUNs from the array, 
so you could create a 2.7TB 8 disk RAID6, then carve off a few LUNs for 
the OS to see.

By the way - FreeBSD isn't the only OS without the addressing necessary 
to support >2TB SCSI LUNs.

> My solution is to use two disks as a mirror, and use the other six for a
> raid6 array, thus losing 3/8 to raid overhead instead of 4/8, but it's
> still worse than 2/8 which is what I wanted to do ...
> Perhaps I misunderstand you ?

That sounds reasonable, and should give you a working setup.  Judging by 
the disks and setup, it sounds like you are looking for a good archive 
type storage, and not high performance (high-IO) storage, so the above 
configuration would suit you just fine I suspect.


Eric Anderson        Sr. Systems Administrator        Centaur Technology
Anything that works is better than anything that doesn't.

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