[lug] RAID installation on Fedora 6 Zod

[steve at badcheese.com]

By flakey when talking about linux's software raid is when a drive fails. 
Since the IDE controller (underneath the software raid) isn't raid-aware, 
if a drive fails, you'll get timeouts and your machine will be either 
super-slow or hang due to HD I/O.  The IDE controller will give those DMA 
timeouts and keep trying to read/write to the drive when it's failing. 
If you pull the bad drive and reboot, then linux will be happy and operate 
in degraded-mode and be ok, just needs some manual intervention.  That's 
mainly what I meant by flakey.

With hardware raid, the drive failure is detected (on our LSI controllers, 
we made a nagios plugin to detect a degraded RAID partition, so we get a 
notification), it's marked as failed and the RAID goes about its business 
in degraded mode until maintenance is performed with no perceived impact 
on performance to the user.

- Steve

On Thu, 10 May 2007, Sean Reifschneider wrote:

> Date: Thu, 10 May 2007 21:54:37 -0600
> From: Sean Reifschneider <jafo at tummy.com>
> Reply-To: "Boulder (Colorado) Linux Users Group -- General Mailing List"
>     <lug at lug.boulder.co.us>
> To: "Boulder (Colorado) Linux Users Group -- General Mailing List"
>     <lug at lug.boulder.co.us>
> Subject: Re: [lug] RAID installation on Fedora 6 Zod
> 
> On Wed, May 09, 2007 at 10:42:08AM -0400, steve at badcheese.com wrote:
>> I know that the Linux software raid can be a little flakey, but in my
>
> Flaky?  How so?  For our hosting customers doing RAID-1 on 2 discs, we
> recommend doing Linux software RAID.  It's been EXTREMELY reliable for us.
> Performance is great, overhead is not noticable, and tools to interact with
> and manage it are fantastic.  Software RAID is pretty much the only option
> that installs a tool to alert you to a drive failure as a standard part of
> the OS install.
>
> The most important part of a RAID install is, of course, making sure you
> get alerted when the first drive fails.  Because you *WILL* get alerted
> when the second drive fails, and you won't be happy.
>
> And a hardware RAID controller is no guarantee of fewer bugs or more
> performance.  For example, the highest end 3ware RAID cards do not
> interleave RAID-10 reads across both drive pairs...  RAID 0 and 1 do not
> involve any computation overhead, so offloading to hardware doesn't help as
> much as RAID-5.  And there can be other more subtle problems with hardware
> RAID, like not properly committing the changes to disc before removing them
> from the battery backed RAM.  Read the livejournal outage report when they
> lost power last year for more information about these problems.
>
> I'd recommend you do a new install and use the RAID setup in the installer.
> You can do it after the fact, but if you don't already know how to do it
> I'd guess you aren't quite ready to be doing it.  As far as slapping a
> hardware RAID controller, you need to know if the controller stores data on
> the drive (being invasive) or not.  If it is invasive, you can't just have
> it RAID the existing drive.  Again, you probably just want to do a
> re-install.
>
> Sean
>

-- 
EMAIL: (h) steve at badcheese.com  WEB: http://badcheese.com/~steve