Too Busy For Words - the PaulWay Blog

Mon 17th Jul, 2006

Crazy LVM Partitioning continued

Today, I decided to finally attack the root partition on my MythTV box. By "attack", here, I mean "move off the 40GB drive", or at least do as much of this process as I can. After a bit of a think, what I wanted was a RAID5 array, but LVM doesn't offer RAID. MD does, but all the space on the drives in question is used by LVM. Presto chango, I pvmoved a bit of space around until I had 8GB of space free on each drive, then I made three logical volumes (called, unoriginally, lv_root_1, lv_root_2, and lv_root_3) and created a RAID5 device across them with mdadm -C /dev/md1 -l raid5 -p ra /dev/vg_storage/lv_root_*. This time I kept the mkfs parameters fairly standard; then I mounted it and started copying the root directory across with cp --preserve=all -rxv / /mnt/test.

The thing that most impresses me about this process is that the copying is taking about %5 (on average) CPU for the actual cp process, and barely any time at all for the md1_raid5 and kjournald processes. So doing RAID5 in software certainly doesn't require a huge grunty CPU (this is an Athlon 2400, yes, but it hasn't even broken into a sweat yet). This'd all be possible on a VIA EPIA motherboard... And, once it's finished, I'll have a root volume that can stand a complete drive failure before it starts worrying, and when it does I'll simply add a new drive, create a new PV, add it to the VG, create a new LV for the new drive, add the drive as a new hot spare, and remove the faulty LV; all at my leisure.

The fact that I can understand all this and think it 'relatively simple' gives me a small measure of pride. One of the few things I would thank EDS for in the time I spent there was sending me on the Veritas volume management training course. LVM is still pretty easy to get a grip on without that kind of training, but it's still made things a little easier.

(Educated readers would be asking why I specified -p ra rather than using its default, ls (or, in other words - why use the parity write policy of right asymmetric rather than left symmetric?) There's no particularly good reason. Firstly, I want asymmetric rather than symmetric to spread the parity load across disks, as is consistent with RAID5. Secondly, when I see an option like this I tend to want to choose the non-default option because, if all options are tested equally but most people use the default, then if a failure mode comes up then it's more likely to be found in the default case, and that may not affect the non-default case. It's a version of the "all your eggs in one basket" argument. I don't give it much weight.)

And all of this over an command line, through SSH, to home. I love technology (when it works...)

Last updated: | path: tech / fedora | permanent link to this entry

CLUG Perl Programmers SIG = waste of time

A quick note about last Thursday's SIG meeting of the CLUG. This month was a special Perlmongers theme, given that we're trying to work with the Canberra chapter of the Australian Perl Mongers. So it was a pity that not only does no-one there other than me show any interest in Perl, but one guy (who I won't name) actively calls Perl "executable line noise". Yeah, thanks. That'll really make people enthusiastic about talking about Perl.

What I thought was ironic was that said Perl detractor has been working on some cool stuff - Microsoft's PDF-alternative format, and some other stuff in KDE that was more than nominally neat but has now slipped off the bottom of my stack. Now, I know that I tend to spend a lot of time telling people about my opinions on stuff - this blog may be a prime example. I do also try my best (I think) to be interested in other people's projects and interests, so I don't think I present a one-sided write-only approach. (As always, please email me at, anonymously if you want, if you want to correct me on this :-) It's a little depressing, though, when no-one wants to talk about the topic of the night but wants their own ideas to be heard and applauded.

(Not that I hold anything against the person in question. I just wanted to talk gnarly Perl stuff, that's all :-)

Last updated: | path: tech / perl | permanent link to this entry

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