Linux LVM2 and md arrays

For some time now, I've been running a debian server with several disks in a RAID5 md array with LVM2 layered over the top. At the time this seemed like the usual path that people were taking; ie. Use the md tools to create your redundant drive array, and use LVM2 over the top to get a flexible volume management system. Coming from a Veritas Volume Manager background, this always seemed a bit dodgy (Veritas manages the redundancy and the logical volumes). However, this setup has proved reliable and straight forward to work on. Admitedly I haven't had a drive failure, but I've never had a situation where the LVM2 or md information needs some form of repair.

I still don't mirror my root filesystem ... but thats a personal choice.

At the time I setup this disk array, there were some notes indicating that the Linux device-mapper stuff (which effectively implements the core of LVM2) would implement mirroring. It looks like that is the case now, though it's not exactly mentioned in many places on the net. The main resource seems to be redhat who I think maintain the LVM2 stuff.

Anyway, you can now do mirroring without an underlying md array. It's really quite simple. If you have another drive you need to pvcreate at least a partition on it and then vgextend and finally lvconvert to do the actual mirroring. Here's an example from an LVM'd debian setup

pvcreate /dev/hdd1
vgextend debian /dev/hdd1
lvconvert --corelog -m1 /dev/debian/usr

Not too sure what the corelog does. I kept on getting an insufficient extents error until I added it. You can do a 'lvs' to see the progress of the mirror. It works pretty well. Like most LVM operations you don't need to umount anything while they're running.

But you still need to use md if you want RAID5. I don't think the device mapper implements RAID5 yet. One thing that is good about current (post kernel 2.6.16) md setups is that you can add a disk to a RAID5 array and dynamically expand it (google for mdadm raid5 reshaping). If you understand how RAID5 works, you'll realise that adding a disk is a complicated process involving restriping data across all the disks in the array. When I set up my array this reshaping feature was not available so I deliberately split each drive up into about 5 or 6 partitions to create multiple RAID5 instances .... which suffice to say allows me to add another drive and reshape it myself (it's a long process, but doable)

Compiling kernel 2.6.20 for a Core Duo

Am just trying out the latest 2.6.20.6 kernel on Debian Sarge, and the first problem I hit was having the 'make bzImage' fail very early on with an error similar to:

cc1: error: invalid option `tune=i686'
make[1]: *** [arch/i386/kernel/asm-offsets.s] Error 1
make: *** [prepare0] Error 2

It turns out you need to use gcc 3.4 or higher if you choose Core Duo as the processor type. There are gcc-3.4 debs in the normal repositories for Debian Sarge.

ADSL speeds

Just a quick note about ADSLv1 speeds. Here in NZ, ADSL2 and ADSL2+ are really just wishful thinking, so we're stuck with ADSLv1 at the moment. Its not too awful. Its essentially; up to 8Mbps down and up to 800kbps up depending on distance to the exchange and bunch of other factors. And actual throughput beyond your exchange is highly variable depending on how congested your exchange is.

For 4 or 5 years I've used an Alcatel Speedtouch USB modem attached to some form of linux box (more recently to an OPENWRT box). It has served me well, but has never managed download speeds higher than 2.3Mbps. Having had a few problems with my ADSL connection lately, I tried using a Dynalink RTA1025W ADSL2+ capable modem on the line, and the speed difference is staggering. This new ADSL router will max out at around 6Mbps ... which is rather fine and dandy.

The Dynalink is an interesting device. It seems to run some form of Montavista linux, and you can even enable SSH access via the web interface. Its a typical little busybox/uclibc type of setup. The only problem with the command line access is I can't work out how you save anything. This would be nice so you could add your own iptables rules and other stuff. One thing that I would like to work out is how to configure and save changes to the 4 port switch. It looks very similar to the one in the WRT54G devices whereby you can configure it to effectively look like multiple interfaces and hence have a true DMZ. The web interface kind of hints at the fact that you can do this but doesn't really tell you how.