[Tfug] Software RAID suggestions

[Harry McGregor]

Tom Rini wrote:
> On Thu, May 14, 2009 at 10:57:36PM -0700, John Gruenenfelder wrote:
>   
>> One more question about the machines I'm building...
>>
>> This concerns software RAID.  For a machine that does not need to maximize
>> drive throughput (i.e. it's primary purpose is *not* a file server), is there
>> any reason *not* to use software RAID in the Linux kernel?  I've been using it
>> for years at home and it's always been great.  And it seems to nicely avoid
>> the problem of incompatible RAID formats one gets from RAID hardware.
>>
>> I ask this because this new machine will likely have two 1.5TB drives running
>> in a RAID-1 mirroring configuration.  Most all new motherboards have some sort
>> of RAID hardware built into them, but I had planned on using the kernel's RAID
>> facility instead.
>>     
>
> I assume Harry will chime in for the most part, but the "RAID hardware"
> you find on most PCs isn't really.  It's what Linux calls "fakeraid",
> and is roughly software RAID that the BIOS recognizes, with no real
> hardware assist.
>
>   
Tom is VERY correct.  The "motherboard raid" is basically a bios hack
that gets you to the windows or linux driver.

You get no real hardware assist.  The only times these can be useful for
linux are if you need to boot a raid-5 which I rarely if ever suggest.

-----

Linux software raid is quite stable, and easy to debug if your a linux
geek.  It's even relatively easy (ask Nick) to fudge the metadata if
needed when you have many drives go offline at about the same time (Nick
and I "lost" 3 out of 6 drives in a raid 6, due to seagates firmware bugs).

So as far as data recoverability, Linux software raid rules.  Also in
most cases it will be faster than hardware raid.  Unless you are careful
to make sure that grub is installed on both drives (and thinks that both
drives are (hd0) when you install it), you may have issues booting with
a drive failure.  Keep a grub boot disk/usbdrive/cd handy just in case

If you are concerned more about the OS being un-aware of the issues, and
the controller just booting no matter what, true hardware raid is the
way to go.

-----

If you need VERY high end raid performance, look at a dedicated SAN
solution, and even SAN boot.  But this is out of the budget of most
people on this list.

Check out www.xivstorage.com


                            Harry