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[#] Tue Aug 04 2009 10:31:09 EDT from IGnatius T Foobar @ Uncensored

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Typically, LVM sits on top of RAID, not underneath it. These are technologies that solve different problems. LVM is about managing your storage space, while RAID is about protecting your data from drive failure.

The one place they overlap is RAID 0, which joins together two or more drives and presents them as a single drive without any mirroring. Unless you require the bus bandwidth distribution of RAID 0, LVM is usually a superior way to go for this because it doesn't require all drives to be the same size.

LVM never protects your data from hardware failures. That's not what it does. The purpose of LVM is to disassociate your volume size requirements from the capacity of your physical media.

If you want to play the "never touch this again" game, RAID is still the way to go. There are some reasonably affordable disk controllers out there that will do RAID 1 (simple mirroring) of IDE or SATA drives. You can also do software RAID, which in Linux is quite reliable, but keep in mind that if you do lose a disk, there are a couple of steps required to get up and running again after you replace it. With hardware RAID you just pop in a replacement disk of the same size and turn it back on.

If you're building a new disk system, you'll still want to go with LVM on top of RAID just to have the flexibility it offers. The standard disk layout of a RedHat/Fedora/CentOS installation, if you let the installer decide how to partition, reflects the current best practices: set aside the first 100 MB or so for /boot (because LVM doesn't go active until after the kernel has initialized) and then the rest goes into an LVM PV. Over that, they set up a single VG containing your root filesystem and swap partition.

[#] Tue Aug 04 2009 15:21:06 EDT from Ford II @ Uncensored

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Yeah, I had a vague cloud view of all that stuff.
But I figured I could be really sneaky and use the technology against itself.
(consider my usual situation of having a bunch of shitty hardware lying around and desiring to do something cool with it.
)

I figured raid wasn't supposed to run on top of lvm, but can it? If it could then I could make the lvm's look enough like identical drives (by splicing together various pieces of shit hardware) enough that the software raid would be able to mirror from one to the other.
I'm sure it would be shit slow, but it certainly would be cool.

[#] Tue Aug 04 2009 15:34:30 EDT from IGnatius T Foobar @ Uncensored

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Theoretically you can run anything on top of anything else in Linux ... but to make a configuration like that come up at boot time, you'd probably have to modify a bunch of the system boot scripts. The RAID subsystem is initialized before the LVM subsystem.

With the rising cost of electricity, I wonder how quickly the cost of a newer, bigger disk drive would be made up for by the cost savings of operating one drive instead of two.

I'm planning to eventually rip the BiggieDisk(tm) out of my old TiVo and put it into my Linux machine as a place for backups ... and configure it to automatically spin down when it's not in use.

[#] Tue Aug 04 2009 17:14:22 EDT from Ford II @ Uncensored

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I can't imagine nowadays that drives consume all that much power, even the 40 giggers, they're not that old.
And it just kills me to have all theses little drives around and have to go out and buy a big drive because I can't use them.

Interesting project, I guess, but not worth it.
Which probablyu means some boob on the web has already done it, and I just have to find his webpage.

[#] Tue Aug 04 2009 17:16:11 EDT from Ford II @ Uncensored

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[#] Wed Aug 05 2009 10:24:12 EDT from dothebart @ Uncensored

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so I guess laughing could be finaly stated as closed past:

http://www.cio.com/article/print/498963



[#] Wed Aug 05 2009 18:04:31 EDT from IGnatius T Foobar @ Uncensored

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Perhaps. Or perhaps they're simply playing the "look, we're not a monopoly after all" card.

[#] Wed Aug 05 2009 18:54:21 EDT from dothebart @ Uncensored

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their biggest competitor still is pirated windows by magnitudes, I know.



[#] Mon Aug 10 2009 16:49:51 EDT from Ford II @ Uncensored

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So I've given up on the raid over lvm because I don't have enough parts to make it go. But it's still a neat idea.
I've decided instead to try the software raid on my boot drive.
I found a drive the same size as my boot drive and if I understand this right, I make a partition the same size as the boot partition (I don't have /var /usr /boot separated out) and tell mdadm to make a raid array and then tell grub to boot //dev/mda1 or whatever its called.
Apparently setting it up from scratch isn't too hard, but adding raid to an existing setup can be not simple.
You make the array with a space for another device then add it, then sync to it and hope you don't do it in the wrong direction or the blank drive synces over your real drive.
Anybody ever play with this, have any tips?

[#] Mon Aug 10 2009 17:24:01 EDT from IGnatius T Foobar @ Uncensored

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Generally the way you migrate to RAID is to start with a partially-configured set (declare one drive as broken) and then migrate it, then bring the other drive into the set using raidhotadd.

Try this:

http://alephnull.net/software/linux-raid/7%20-%20FAQ.html

[#] Mon Aug 10 2009 20:28:20 EDT from Ford II @ Uncensored

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Cool, thanks.

[#] Mon Aug 10 2009 20:49:47 EDT from Ford II @ Uncensored

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wow that's scary, I have to blow away my old drive's partition table? yeesh.

[#] Mon Aug 10 2009 22:18:40 EDT from IGnatius T Foobar @ Uncensored

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I didn't say it was safe; I said it was possible. :)

(I did do it once, a long time ago. But I had full tape backups too.)

[#] Tue Aug 11 2009 20:42:09 EDT from Ford II @ Uncensored

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I just had an odd thought.
Since you can mount a file as a mountpoint, couldn't you do raid from one drive to a file on another drive (assuming there was enough space for the file?)
Wouldn't that be interesting.

[#] Tue Aug 11 2009 21:19:12 EDT from Ford II @ Uncensored

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well as the saga unfolds....
I found out that the drive I was goign to raid to was .8gig short of the drive I was raiding from.
so I had to shorten the filesystem and partition and move my swap around, but it worked.
As in the machine still boots.
It's low level and geeky, but it's such old technology that it just works.
I'm impressed.

[#] Tue Aug 11 2009 21:20:55 EDT from Ford II @ Uncensored

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cool. I just copied the partition table over, and it just worked. frikkin cool.

[#] Wed Aug 12 2009 02:26:09 EDT from dothebart @ Uncensored

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always leave some extra hundret MB's at the end of disks you're raiding...

the replacements you're bying might be a little smaller.



[#] Fri Aug 14 2009 17:14:56 EDT from IGnatius T Foobar @ Uncensored

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always leave some extra hundret MB's at the end of disks you're raiding...


Windoze users don't have that luxury ... their "dynamic disk" layout expects to find metadata at the *end* of the volume.

[#] Mon Aug 17 2009 07:02:09 EDT from Ford II @ Uncensored

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So I'm up to the part where I fix my grub config
I don't get it and the man pages aren't helping.
There's a command called "root device [hdbias]"
Has something to do with booting debian or something like that.
but I don't get the naming scheme for 'device'
it's like hd(0,0) or hd(0,1)
but it doesn't explain what those numbers mean.
Any ideas?

[#] Mon Aug 17 2009 07:18:43 EDT from dothebart @ Uncensored

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for that reason I stayed with lilo, as i know how to operate it, and get it through the more complicated cases, in which it is more what one knows from the rest of the linux handling.

LVM and Grub sucks. Grub in general sucks. Damn, that thing just has to fire up the kernel. I've got boxes that won't boot for months. I don't want to make a bootloader diploma before I can handle cerntain stuff.



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