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Re: [linux-lvm] Sanity check: newbie wants strange LVM configuration


"2 Day air" took a week to deliver... but I have my 40GB HD and have started the crossover.

The first thing I missed was that fdisk only sees up to 64K cylinders (and this disk has 77K), so it said I only had 12K... and it wasn't until I created the first lv that I realized the problem. I was able to back-out the LV with lvreduce. vgreduce was not as kind: it wouldn't let me remove the last PV from the VG. So, I repartitioned using cfdisk (which could see more than 64K cylinders), and every lv/vg command from that point on would say that the configuration file and the kernel representation were out of sync, and to run vgscan to get them to jibe. But, vgscan wouldn't correct the problem: I had to reboot. I still can't create a new VG with the same name as that first one I created (so I created one with a different name). Is deleting the VG as simple as just deleting the device file with the name of the VG?

Anyway, I made my first temporary 12GB Reiserfs from an LV, and have copied the data from my system drive (this is the first step in converting the current system over to LVM: copy the system drive to a temporary LV... next, I'll boot from the temporary LV and make the old system drive part of the LV, copy the data back, and boot from the new LV).

I used bonnie++ to test the different disk's speed: they're all about the same: 1.6MB/sec character I/O, 5MB/sec block I/O (the file I/O was significantly faster with the new disk running the reiserfs -- I figured this was the reiserfs and not the disk). I thought hanging the new disk on the same controller as the cdrom would effect it, since my IDE controller doesn't do UDMA. No effect. I bought the cheapest disks, they're as expected: slow. Time for software to speed them up.

Now that I've got the system backed up, and am ready to try booting from the lv (as per the booting from lv root instructions), I do have another question: the lv, which is basically on hdc3, is about to move to hda3 (the new bootable partition is on this 40GB drive). "pvmove" doesn't seem to be the right way to do this, since I don't want to copy data from one disk to another. Will vgscan automagically figure out the cabling change at boot?

Joe Thornber wrote:

LVM is a bit more flexible in it's striping than MD. You will create
physical volumes, each physical volume will be split into physical
extents (4M by default), it's only when you come to create a logical
volume that you tell it whether you want to stripe or not. At this
point the extent allocator trys to allocate extents from different
PV's. So there is no need to create a 24G striped volume for only 12G
of data.

So, in the end I'll have 3 PV's (40GB, 16GB, and 12GB) in a 2-way striped LV. Does lvm automagically know to stripe the 12GB and 16GB with the 40GB, and not with each other (I'll be cabling my IDE controllers for this configuration)? With MD, I'd specify which disks are striped together, in lvm, I don't see a way to do this.

Furthermore, I want the last 12GB of the 40GB drive (that has no matching disk to be striped with) to be linearly appended to the end of the resulting lv. MD's can be made of other MD's -- so I could specify two raid0's (comprised of: the 12GB drive + 12GB from the 40GB drive and: the 16GB drive + 16GB from the 40GB drive) then linearly append them together in a third MD with the leftover 12GB (unstriped) from the 40GB drive at the end.

But, VG's can't be comprised of other VG's -- and LV's are either striped or not.

How does LVM accomplish this same task? What am I not understanding about LVM?

Let me know how you get on, writing a decent HOWTO is on my todo list so these sort of problems are useful to me.

You wanted the strange questions. I got 'em ;)


P.S. I'm having a hard time sending email from this address to the LVM list server... I'm on the mailing list from another address, and when I send from this address I get a reply that the admin will review my email before posting, but it never happens. Feel free to post your reply (and copy my stuff) to the list server. I don't mean for this to be a private post, but I don't want to subscribe to the list for every email address I use.

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