Too Busy For Words - the PaulWay Blog

Or: Linux Disk Craziness continued

I'm reconfiguring my MythTV server so I can remove the 40GB disk - it's old, it's dwarfed by the other drives and I'd like to have one IDE channel back for the DVD drive. This involved a bit more fun with LVM, so I thought I'd document it...

The first task was building a non-LVM /boot partition. That was accomplished by using pvresize. I wanted 100MB, but the LVM tools display size in GB, allow you to set it in MB, but round up to an extent size. I played it safe and reduced it by three times as much as I thought I'd need. Then I used fdisk to resize the LVM partition and add a new partition. First challenge: fdisk doesn't allow you to modify a partition, you can only delete it and then create it anew. Luckily I remembered to change the type of the new (LVM) partition to 8e. Second challenge: fdisk only allows you to specify partition sizes in number of cylinders or sectors. So a couple of quick back-of-envelope calculations later, I had something which was roughly the right size. I then formatted the new partition, using -i 16384 because /boot contains relatively few files that are relatively large - this saved a relatively trivial 3MB on copying. I then used pvresize to expand the LVM area back to its maximum extent inside the partition. All done.

The second task was creating a swap partition. Because I'm a speed-mad power freak, I wanted a stripe across all three LVM physical disks. Ooops, the current LV already takes up all free extents on the first two disks, so I go to work with pvmove. After I remember to specify /dev/hdc1:(starting PE)-(ending PE), rather than /dev/hdc1:(starting PE)-(number of extents), this works rather nicely. Then it's a simple matter of swapoff -a, lvcreate -S 1G -i 3 -n lv_swap vg_storage, mkswap /dev/vg_storage/lv_swap, vi /etc/fstab to change the swap device, and swapon -a to get it working again. That's the easy part. :-)

The third task is to create a root partition. Hmmm, slight problem: There's not enough space in the volume group for a 20GB root partition. Further problem: the storage LV in the main VG is formatted as XFS, and XFS can't be shrunk (at the moment). OK, I'm going to have to think about this one. But, driven mad by power now, I consider how I'd configure it: how about a mirror of two three-disk-striped LVM arrays? At first blush it sounds reasonable. How fast would it be? I set up two 1GB LVM partitions, and, delving into mdadm, create a new /dev/md1 as a RAID1 mirror across them (the full command was mdadm -C /dev/md1 -l mirror -n 2 /dev/vg_storage/lv_test_1 /dev/vg_storage/lv_test_2). time dd if=/dev/zero of=/mnt/test/thing count=200000 (displaying my great ability to choose meaningless names) reports 2 seconds to write a 100MB file. That's pretty good, I reckon.

Of course, if any one disk goes down, that does take the whole thing with it, which is not exactly the required effect. But I'll remember that bit of the Veritas Storage Manager course sooner or later, and in the meantime I have larger fish to fry...