Mirror Mirror

[Scott Mertens]

On Thu, 2005-04-14 at 14:35 -0700, Rick Stevens wrote:
> Scott Mertens wrote:
> > Scott Mertens wrote:
> > 
> >>With these SATA drives as cheap as they are I am thinking about just 
> >>purchasing another 200 GIG HD and setting up a mirror in Linux.
> >>
> >>
> >>My thinking is that if one drives fails, I can simply break the 
> >>mirror, switch SATA hard drive cables - re-boot and away I go.
> >>
> >>I am using
> >>Red Hat Enterprise Linux ES release 4 (Nahant)
> >>
> >>Can anyone tel me if this is possible, or do I have my head way up my 
> >>???  Is this something Linux can do out of the box, or is there some 
> >>add-on's I need to accomplish.
> > 
> > 
> >>It is certainly doable by either hardware or software.  You're talking
> > 
> > about RAID-1 (disk mirroring).  Most SATA >controllers are capable of doing
> > it, but usually need the help of the OS to accomplish it.
> > 
> > 
> >>Linux has software RAID (called "md" for "multiple devices").  grub has
> > 
> > support for booting software RAID 
> > 
> >>>>>>volumes. I'd recommend you go off and check out the Software RAID HOWTO
> > 
> > at:
> > 
> > 
> > Thanks Rick, good advice.  I'm wondering if this might be a better way, as
> > far as getting back up and running quicker and easier than say rsync. Drive
> > space is pretty inexpensive.
> > 
> > Any thoughts on ease of use/setup of either method?
> 
> rsync will synchronize files, but it won't synchronize your MBR.  If
> your primary drive croaks, you won't have a bootable system on your
> backup drive.  rsync was really intended to distribute things like FTP
> repositories, websites and things like that.  You've heard of FTP
> mirrors, right?  That's done by rsync.
> 
> RAID-1 is the cheapest fault-tolerant system around.  Every time the
> primary drive gets written to, it is duplicated on the mirror drive.
> If the primary goes teats up (pardon the phrase), the system fails over
> to the mirror drive and your system stays up.
> 
> When you replace the failed drive, however, you have to tell the system
> to rebuild the array so the new drive reflects the operating drive.
> This takes time to do and the system will run slower while it occurs.
> Some systems won't let you even make the RAID array writable until the
> rebuild occurs.  In other words, the machine will continue to run
> despite a disk failure.  However, when you replace the drive and bring
> the machine back up, it may not come up fully until the RAID has been
> rebuilt.
> 
> There are inherent dangers in any software RAID.  If something corrupts
> the RAID software (bad RAM, some malignant application, a CPU overheat,
> etc.), then the RAID is vulnerable.  In our operations, critical systems
> use hardware RAID, but you need a special controller to do it.  The SATA
> controllers on most motherboards, while claiming to be hardware RAID,
> are in reality somewhat akin to "winmodems" in that the hardware is
> capable of doing a lot of the RAID-1 stuff but needs some assistance
> from the operating system to do the whole thing.
> 
> Understand that what we do is WAY above the average person does.
> Because of the nature of our business, we use multiple redundancy.  All
> of our critical systems are load balanced (there's at least two machines
> that do the job) and all of them run hardware SCSI RAID controllers and
> run them in a RAID-5 format (minimum of four disks--three for the RAID
> and a hot spare).
> 
> If we lose a drive, the controller brings up the hot spare and rebuilds
> the RAID on-the-fly. The system stays up--even while the RAID is being
> rebuilt.  We can then replace the failed drive and the new drive becomes
> the hot spare.  Note that the Adaptec 2100S (a fine hardware RAID
> controller) costs about $400.  A 73GB SCSI drive probably costs about
> $300, so one of our systems has about $1600 invested in its disk array.
> However, barring catastrophic issues, they're pretty robust.  I've yet
> to have one of the 120 or so of those systems go down in five years due
> to disk problems.  CPUs, yes.  Power supplies, yes.  Memory, yes.
> Disks, no.
> 
> If a machine dies because of something else (power supply, etc.), the
> load balancer routes traffic to the surviving machine that does the same
> job.  It takes a lot to kill off a service we offer!  BTW, this sort of
> thing is technically called "RAIM" (redundant array of inexpensive
> machines).  We call it "LOCH" (lots of cheap hardware).
> 
> All that being said, for what you (and the majority of home users) want
> to do, a software RAID-1 is peachy and will work very well.
> 
> NOTE:  A RAID system is NOT a replacement for devising (and using) a
> good backup strategy.  RAID helps your machine survive hardware faults.
> Backups will save your sanity--even if it's something as stupid as "Oh,
> sh*t!  I didn't mean to delete THAT file!"

Great info.  I may be going overboard for a home network, however I have
suffered 2 critical failures in the last 5 years that have cost me
greatly.  3 years ago home struck by lightning, charge entered through
modem and fried my PC.  and a HD failure that was not backed up.  Since
I run a business from my home now I can't afford the downtime.

I use ghost on two laptops and one desktop that belong to wife and kids
using Windz.  It to has it's limitations.  In case of failure mostly I
have to restore to the same machine or very similar hardware for it to
work. I have taught everyone to save data on their win machines to the
My Docs folder and once a night a script backs those folders to a
network drive.  So worst case I have to reinstall software, but data is
always in 2 places.  So just kind of looking for the same redundancy for
the new Linux box as I am learning about it.

I'm not sure but someone told me Ghost cam image a Linux box as well.
Not sure how to accomplish this as Ghost is a Windows program?  Maybe
must have dual boot set up, boot to win and run Ghost on the linux
partition? Or somehow share the Linux HD and pull the image across the
wire at night? Don't know may have to research some more.

Thx again.