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Debian 6 "Squeeze" MD RAID1 Notes

2015-05-04

Recent exeriments centered around using multiple disks on a Debian 6 "Squeeze" ssystem have proven very educational and successful.

My primary purpose was to learn about the use of software RAID capabilities available in Linux/Debian and how that type of facility might be beneficial to me as I manage my own local server. I have been concerned lately about the possible data loss, since more and more data is going onto these servers, including digitial photos, music, movies, and a growing body of archived emails, and my 'diary' of doings with my antique computing gear, via my antics pages and my personal, private wiki. All of these elements are spread of multiple servers at present, and I believe the time has come to consolidate it all. However, if it all goes onto one server, more or less, then I had better be ready for the inevitable hardware failure, either the host CPU, or more likely, the hard disk.

Many experiements were performed to educate myself about how a mulitple drive RAID array might serve as a way to eliminate, or at lease minimize, the danger of loosing a/the hard drive to a failure. And, though I have successfully used modestly powered laptops as my servers for the past 13 years, they only have a single drive each, and this means a single point of failure for the storage system. My new plan is to use external hard disks, through USB connectivity, that can easily be added, removed and possibly relocated to a different CPU if the host box fails. In this way, I hope to have a pretty robust, reliable, but flexible, and manageable system that I can adapt as my needs grow or change without having to resort to building up a fresh system each time a change is required.

So, I have learned about the new (to me) software RAID facility in Linux, and in the course of these experiments I have also learned about modifing an array, moving the data to bigger drives, and handling a disk failure. I have also leaarned about ways to manage the power to external USB-attached drives, that is, establish the means to power them down when the system is idle.

So here are my key findings:

Linux (and therefore Debian) since the version 2 kernel has a software-based RAID capability, that results in the creation and use of /dev/md devices. The software raid drivers put a layer between the kernel and the drives themselves. The RAID array is made up of hard disk partitions (/dev/sda1, etc.) and presents a device (/dev/md0, etc.) to the kernal. So the md devices are mounted and manipulated more or less as the disk partitions were in the past.

There a number of resources online that describe how to setup a RAID array at the time of installation of the OS, but I was interested in learning how and if it was possible to build up a RAID array on a pre-existing system. It turns out that this is possible and I found a particularly helpful How-To on HowToForge.com:

The key is to follow that article's recipe (carefully!) and it results in a RAIDized system with mirrored drives, without a reinstall. Two reboots are required.

On top of that matierial, I learned about increasing a RAID array's storage capacity, that is replacing the presently used disks with larger ones, and extending the filesystem accordingly. This can be done with NO REBOOTS, amazingly!

Lastly, there is the issue of how to cause these USB-connected external drives to spindown when idle. Turns out that the traditional tool hdparm is not useful here. It was designed to work with ATA drives. USB-connected drives (though they may be IDE (PATA) or SATA drives inside the enclosure, are treated as SCSI devices by the kernel and and as such hdparm is not useful. A tool called hd-idle is the solution.:

hd-idle is well known, but for some reason has not been packaged for Debian in the repositories. Using the information from the project's page, it is possible to download the source and create a Debian package (this is what I did) that then can be managed using the ordinary dpkg and apt tools.

This tool installs itself into the init.d subdirectories and is run as a deamon using the normal methods, e.g., /etc/init.d/hd-idle start, etc., and it is setup to be launched at boot time via the /etc/rcX.d runlevels. It can handle a global timeout, and/or a timeout for individual devices.

Mirrored disks via software RAID

  • follow the recipe in the How-To. When building RAID arrays using external USB devices, beware when the device assignments change, e.g., /dev/sdc becoming /dev/sdb depending on enumeration order from boot to boot, and whether or not the device is recognized as a valid Linux device at the time. I had to adjust the steps a bit to account for this, particularly as it concerns GRUB and how it boots. I ended up learning about the GRUB command line and modifying the GRUB configuration installed on a device in order to overcome the device assignment mixups. BEFORE INSTALLING GRUB, ZERO OUT THE FIRST 63 SECTORS, otherwise GRUB can get messed up and won't boot, due to previous contents. If forgotten, zero, then reinstall GRUB a second time!
  • IT IS IMPORTANT TO TELL GRUB TO DELAY KERNAL STARTUP to allow the USB devices to be discovered and enumerated or boots from the devices will fail, once the raid ARRAY becomes the bootable device. I solved this by putting:
    • rootdelay=xx (usually 10)

on the kernel command line. This can be done by interrupting GRUB and editing the menu entry as needed while building the RAID, then modifying /boot/grub/grub.cfg when RAID is up and running. Then doing a grub-install, of course, to each of the devices (/dev/sda, etc.) that constitute the bootable array.

Replacing the array with bigger disks

  1. follow the recipe. This turned out easier than I thought it would be.
  2. IMPORTANT: if a 'new' device being integrated into the array has once before been part of an array, it must have it's superblock erased or the mdadm will not be able to increase the 'allowed' size of the array:
    1. mdadm --zero-superblock /dev/sdb1 (for example)

Building and installing hd-idle

  1. follow the project's recipe on it's home page to compile and build the debian package. This required that I install two key packages from the normal Debian repositories and associated dependancies:
    1. dpkg-dev
    2. <mumble>
  2. The question is: Now that I have the .deb package, can I just archive that and use it on any system without rebuilding? So far, this has proved true between "Squeeze" systems. I don't know if it will work on other versions of Debian.