Re: [PLUG] LVM primer (Formerly: "reinstalling grub")

Art Alexion <art.alexion@verizon.net> · Mon, 18 Aug 2008 11:29:19 -0400

Thanks, Matthew.  I had found the How-to you cite.  Turns out my problem may 
have been related to a flaky BIOS chip on the motherboard.

At some point I gave up on trying to reinstall grub.  The new strategy was to 
add another drive do a clean OS install on the new drive, and copy the data 
and home back to the new drive, using the old drive as a backup system.

I had trouble doing this because once I installed the new drive and moved some 
others around, the system refused to complete a POST.  I had been having 
other drive problems with this computer with drives intermittently 
disappearing.  The CMOS seemed to corrupt every time I made a change to it or 
it automatically changed due to a hardware change. 

I fixed it a couple of times and finally MSI, the board manufacturer 
acknowledged that it was a faulty BIOS chip.  They told me the reseller, 
newegg.com should replace it under the warranty.  Wish me luck.  NewEgg is 
not known for warranty service.

How was Belgium?

On Monday 18 August 2008 10:30:44 am Matthew Rosewarne wrote:
> On Monday 28 July 2008, Art Alexion wrote:
> > Part of my problem is not fully understanding LVM.  Is there a good
> > primer that you can direct me to?
>
> Sorry for the late reply, I've been busy with Akademy.
>
> The short version:
> 		(technology)
> 	Hard disks and most other storage devices are called "block devices"
> becuase data is read/written in "blocks" (usually 512 bytes for HDDs, 2048
> for CDROMs).  This is different from "character devices", which read/write
> one byte at a time.  Without LVM, the disk is divided into partitions,
> which are specified as a start position (in blocks) and a size (in blocks).
> 		(analogy)
> 	Imagine writing a novel on a long tickertape.  You need to chop the tape
> into chapters before you've actually written anything.  Hopefully you've
> left enough space for your chapters to fit, since you've already cut up the
> tape and thus can't change any chapter's start position without starting
> again with a fresh tape.
>
> 		(technology)
> 	LVM is an abstraction above disks and partitions.  You designate some
> block devices as "physical volumes" ("PV"s for short).  Instead of writing
> directly to these devices, they are divided into "extents", which are much
> larger chunks of data (4MB by default).  PVs are added to "volume groups"
> (or "VG"s), which tie them together into one big pool of extents.  "Logical
> volumes" ("LV"s) are created by allocating a number of extents from the VG.
>  Using the kernel's "device mapper" functionality, LVs show up as just
> another block device, so they can be used just like actual partitions. 
> When you read/write to an LV, the LVM driver uses its metadata to map the
> virtual blocks on the LV to the real blocks on the actual device.
> 	The fun comes from the ability to simply add or remove extents from the
> pool, letting you resize LVs at will.  You can even do more interesting
> things like striping extents between PVs for a RAID0-like setup, or using
> "snapshots" where changed extents are written using copy-on-write and the
> original data is preserved.
> 		(analogy)
> 	Back to the novel, this would be like taking that tickertape and chopping
> it into many smaller pieces and putting them all in a bag.  You reach into
> the bag and grab some pieces to write a chapter.  If this chapter grows too
> big to fit on those pieces, you just grab some more from the bag, and
> likewise if you don't need that many pieces, you put them back in.
>
>
> Here's an explanation from the LVM HOWTO:
> 	http://tldp.org/HOWTO/LVM-HOWTO/anatomy.html
>
> There's also a fairly comprehensive explanation of LVM from DeveloperWorks.
> 		http://www.ibm.com/developerworks/linux/library/l-lvm/