Nice page.
Just recently been dealing with a HW card that udev doesn't like (doesn't have
SERIAL or SHORT_SERIAL), so I've been thinking about this a lot recently.
* * *
As for pros v cons...
I would think the main advantage to modern HW RAID systems is the ability to
hot-plug. SW RAID has many advantages, but being able to detect a failure, and
then physically see, replace, and rebuild a degraded array while the array is
alive has absolutely got to be the unequivocally primary benefit of a complete
HW RAID setup.
SW RAID is great. Generally faster (at least for RAID-0, back when I used to
benchmark this sort of thing). But, to be fair, while SW has the benefit of
being open-sourced, it does suffer from version skew, too. I have no idea if
new kernel versions make old MD devices unrecognizable, or if everything is
always backwards-compatible. That's worth finding out & mentioning. And, even
if the kernel is currently backwards-compatible ATM, who's providing the
guarantee that newer versions will also be? Sure, it's open-sourced, but,
realistically, most RAID users aren't going to be able to kernel-hack
(driver-hack) in the event that the kernel eventually deprecates a version of
the MD driver. To me, that's just as bad a problem as not being able to find
the same HW card.
It's also worth saying that in software RAID, you have to shut down the machine
to do any repairs, even if the array is running in a degraded state. Unless
you have PCI- or SATA-hotplug in your kernel (is this widely supported or
stable)...and even then, you'd have to be able to put those drives in a
hot-plug bay.
Might also want to mention hot spares.
And...(again, still trying to be constructive, not a jerk)...a page about RAID
absolutely has to have a recovery HOWTO. It's just dangerous not to include
it, lest someone get a machine running, and has no idea how to recover from it.
And, in addition to the "normal" recovery scenarios, point out how it might be
worth using with udev (disk/by-id) long names lest they reorder devices (or the
kernel does it on a version change). I personally just went through this the
hard way on a colo server...
Also, it's probably worth mentioning that most HW RAID setups (provided they
have a Linux driver) make recovery much easier. Just slide a new drive in,
maybe issue a command or two, and the card's firmware will take care of the
rebuild, all while the machine continues to run. With mdadm, I think the whole
recovery process is harder (or, more involved & dangerous to someone who might
be new).
Finally, might want to combine the RAID and LVM pages.
Q
On Feb 8, 2012, at 4:11 PM, Bruce Dubbs wrote:
> I've added mdadm and a new page, About RAID, to the book.
>
> I'd appreciate feedback on ways to improve it.
>
> http://www.linuxfromscratch.org/blfs/view/svn/postlfs/raid.html
>
> -- Bruce
> --
> http://linuxfromscratch.org/mailman/listinfo/blfs-dev
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