Looking for RAID Drivers

[Brian McKee]

On Mon, Sep 22, 2008 at 4:40 PM, Mark Haney <mhaney at ercbroadband.org> wrote:
> Elizabeth Bevilacqua wrote:
>> On Fri, Sep 19, 2008 at 10:08 PM, John Hubbard <ender8282 at yahoo.com> wrote:
>>> Linux implements its own software raid.  True hardware
>>> raid is pretty expensive and not very common.  Unless you spent a good
>>> chunk of cash on your board, I would guess it is actually software raid.

>> And lest you get concerned about spending money on this card and not
>> being able to use it for RAID - benchmarks have shown that Linux
>> software RAID is faster than most of the RAID cards you can buy. So
>> you'll still be making out fine (or better!) by using the controller
>> as simply an SATA controller and then using Linux software RAID.

> I'd REALLY like to see those benchmarks.  Because I simply don't believe
> it.  Sure, the linux dm-raid is damn good, but nothing beats having a
> controller do the 'package and assembly' work for you off-CPU.  In a
> resource intensive setup S/W RAID can't cut it, except for maybe (and I
> emphasize MAYBE with Reads).

Mark - I hate to argue with you when I can't present benchmarks - but
the processor on most systems today is so fast, and idle so often,
that the dedicated controller really has to be good (read more
expensive) to keep up to software RAID backed by a modern CPU.  From
what I've seen, unless you start needing really serious performance
(read way in excess of what any home or small business server will
ever require) or your server is seriously overloaded, software RAID
keeps up just fine.  Heck, people are running entire extra operating
systems via virtualization because the CPU can run more than one
without appreciably affecting required performance.   And at the other
end of the scale we're talking SAN technology or whatever, where
software RAID is really a moot point.

> When you can offload the work to a dedicated controller you free up the
> CPU for other processes.  The kernel doesn't know or care about the RAID
> functions at that point and can concentrate on what it does best.  Not
> to mention, as per my previous post, that the S/W RAID setup is still
> not as stable as I would like.  And, even on RAID 5 the likelihood you
> lose data is increased with S/W RAID if a drive fails.

Now that's where I have to say Huh?   Why would software RAID be more
likely to cause failure than hardware RAID?
Raid level 1, 5, 10 whatever doesn't come into play here - compare
apples to apples because software RAID supports different levels, just
like hardware RAID does.  I have never seen proof that software RAID
is less stable at the same level.  Got something to back that up?
Especially when we're talking about the half-baked untested RAID they
'throw in' with a consumer level motherboard?   Tell me a quality
battery backed unit is more reliable, and I *might* buy it - but
that's not what we're talking about here.

My beef with hardware RAID is if the controller goes, you're hosed
without a matching controller.  And unless you buy a spare when you
buy the original, you can end up in a real pickle.  It's why I don't
like the Drobo - your data is inaccessible unless you have a backup
Drobo.

As I type this I get the feeling we've had this discussion before :-)
 I'll quiet down now.

Brian