Resizing Linux Logical Volumes

June 12, 2011 under Main

I use Linux LVM as the storage subsystem for Xen virtual machines, for backups volumes, and for organizing storage volumes on NFS servers. It provides great flexibility and performance and the command-line tools included are easy to use.

There is however one operation which I regularly perform that is not straight forward: expanding a logical volume (LV) which contains an ext3 filesystem. The issue here is that while expanding the LV its self is extremely simple, the filesystem contained on it is not automatically resized. So if you simply expand your LV and reboot your virtual machine, you won’t see the extra space on your filesystem.

The procedure for resizing an LV containing an ext3 filesystem is as follows. The commands below will resize the logical volume named ‘Chris’ on the volume group ‘Xen’ (see output from ‘vgdisplay’ and ‘lvs’ to see what volume groups and logical volumes are defined on your system). NB: make sure the filesystem is un-mounted and any virtual machine using it is powered off before running these commands!

# Resize the LV (eg. to 40GB)
lvresize -L 40G /dev/Xen/Chris

# Remove the journal from filesystem (we'll re-create it later)
tune2fs -O^has_journal /dev/Xen/Chris

# make sure it's clean (compulsory - resize2fs will tell you to do this if you forget)
e2fsck -f /dev/Xen/Chris

# resize the filesystem
resize2fs /dev/Xen/Chris 40G

# create a new journal
tune2fs -j /dev/Xen/Chris

That’s it – boot up the VM and enjoy the extra space!

If you want to expand an LV containing a swap partition, the simplest thing to do is de-activate the swap (swapoff), resize the LV, then simply re-format the swap space, for example:

mkswap /dev/Xen/Chris.swap

If you’re using another kind of filesystem and can’t figure out how to resize it, another option (if you have enough space), is to simply create a new LV of the desired size, format it, then copy over your existing data using cp or rsync.

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Trust In Your Hardware

June 2, 2011 under Main

I’ve been building out our capacity for the past 3 years using almost identical hardware configurations: motherboards with Intel Server chipsets, Xeon CPUs, 3ware RAID controllers and Intel NICs.

Sure there have been minor variations along the way: disks get bigger, memory gets faster, CPU clock speeds increment gradually. Overall though, the hardware we’re using today is very similar to what we were using 3 years ago. While some complain about the lack of progress, I am quite happy with this slow-but-steady progress in hardware development. It means I can really get to know the servers I work with every day.

One benefit of this is that I’ve found myself starting to develop a 6th sense for trouble. An I/O op which takes ever so slightly longer than it should, an occasional kernel panic on an otherwise bog-standard Xen virtual machine – small signs that something is just not quite exactly right with the hardware.

Case in point: a new server commissioned on 05/11/2010 which at first glance appeared to be functioning perfectly, but as the days and weeks went by and I started to load it up with production virtual machines, I started to notice the occasional issue which I had not noticed on other servers. Not finding any specific problems, I left it to do its job, and it has been perfectly reliable ever since. Until yesterday, when the RAID controller sent me a half dozen emails telling me one of the drives had bad sectors. Then the drive timed out, degraded the RAID array, shut down then powered back on and came back online. The controller did its job and auto-recovered by rebuilding the RAID array, but now I have solid evidence that in fact something is not quite right with the server after all. Back to the supplier it goes for a thorough diagnosis.

I wish I could pinpoint exactly why I suspected something wasn’t right with the hardware. It would help me identify problems more quickly and efficiently in future. So far I have not been able to come up with anything concrete, it’s just an intuition. The one thing I have learned is to have more faith in my intuition and less trust in my hardware. If something feels off, it probably is.

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