Concatenated Disk Configuration

Concatenated Disk Configuration

It seems like today everyone has a collection of multimedia files. Everything from mp3’s to video clips. I’ve converted most of my audio CDROM collection to mp3’s so I can have all of my music in one centralized location, and not have to hunt down the audio CD with that one song I got stuck in my head. The problem I was faced with is where to store all these files?

When choosing a mass storage solution, the most important factors to consider are speed, reliability, and cost. It is very rare to have all three in favor, normally a fast, reliable mass storage device is expensive, and to cut back on cost either speed or reliability must be sacrificed. In designing my system, I ranked the requirements by most favorable to least favorable. In this situation, cost was the biggest factor. I needed a lot of storage for a reasonable price. The next factor, speed, is not quite as important, since most of the usage would be over a one hundred megabit switched Ethernet, and that would most likely be the bottleneck. The ability to spread the file input/output operations out over several disks would be more than enough speed for this network. Finally, the consideration of reliability was an easy one to answer. All of the data being put on this mass storage device was already backed up on CD-R’s. This drive was primarily here for online live storage for easy access, so if a drive went bad, I could just replace it, rebuild the filesystem, and copy back the data from CD-R’s.

To sum it up, I need something that will give me the most amount of storage space for my money. The cost of large IDE disks are cheap these days. I found a place that was selling Western Digital 30.7gb 5400 RPM IDE disks for about one-hundred and thirty US dollars. I bought three of them, giving me approximately ninety gigabytes of online storage.

Installing the Hardware

I installed the hard drives in a system that already had one IDE disk in as the system disk. The ideal solution would be for each IDE disk to have its own IDE controller and cable, but without fronting more costs to acquire a dual IDE controller this wouldn’t be a possibility. So, I jumpered two disks as slaves, and one as master. One went on the first IDE controller as a slave to the system disk, and the other two where slave/master on the secondary IDE controller.

Upon reboot, the system BIOS was configured to automatically detect the disks attached. More importantly, FreeBSD detected them on reboot:

ad0: 19574MB <WDC WD205BA> [39770/16/63] at ata0-master UDMA33
ad1: 29333MB <WDC WD307AA> [59598/16/63] at ata0-slave  UDMA33
ad2: 29333MB <WDC WD307AA> [59598/16/63] at ata1-master UDMA33
ad3: 29333MB <WDC WD307AA> [59598/16/63] at ata1-slave  UDMA33

At this point, if FreeBSD doesn’t detect the disks, be sure that you have jumpered them correctly. I have heard numerous reports with problems using cable select instead of true slave/master configuration.

The next consideration was how to attach them as part of the filesystem. I did a little research on vinum(8) and FreeBSD’s ccd(4). In this particular configuration, ccd(4) appeared to be a better choice mainly because it has fewer parts. Less parts tends to indicate less chance of breakage. Vinum appears to be a bit of an overkill for my needs.

Setting up the CCD

CCD allows me to take several identical disks and concatenate them into one logical filesystem. In order to use ccd, I need a kernel with ccd support built into it. I added this line to my kernel configuration file and rebuilt the kernel:

pseudo-device   ccd     4

ccd support can also be loaded as a kernel loadable module in FreeBSD 4.0 or later.

To set up ccd, first I need to disklabel the disks. Here’s how I disklabeled them:

disklabel -r -w ad1 auto
disklabel -r -w ad2 auto
disklabel -r -w ad3 auto

This created a disklabel ad1c, ad2c and ad3c that spans the entire disk.

The next step is to change the disklabel type. To do that I had to edit the disklabel:

disklabel -e ad1
disklabel -e ad2
disklabel -e ad3

This opened up the current disklabel on each disk respectively in whatever editor the EDITOR environment variable was set to, in my case, vi(1). Inside the editor I had a section like this:

8 partitions:
#        size   offset    fstype   [fsize bsize bps/cpg]
  c: 60074784        0    unused        0     0     0   # (Cyl.    0 - 59597)

I need to add a new “e” partition for ccd(4) to use. This can be a copy of the “c” partition. Once I was done, my disklabel shoud look like this:

8 partitions:
#        size   offset    fstype   [fsize bsize bps/cpg]
  c: 60074784        0    unused        0     0     0   # (Cyl.    0 - 59597)
  e: 60074784        0    unused        0     0     0   # (Cyl.    0 - 59597)

Building the Filesystem

Now that I have all of the disks labeled, I needed to build the ccd. To do that, I used a utility called ccdconfig(8). ccdconfig takes several arguments, the first argument being the device to configure, in this case, /dev/ccd0c. The device node for ccd0c may not exsist yet, so to create it, preform the following commands:

cd /dev
sh MAKEDEV ccd0

The next argument ccdconfig expects is the interleave for the filesystem. The interleave defines the size of a stripe in disk blocks, normally five hundred and twelve bytes. So, an interleave of thirty-two would be sixteen thousand three hundred and eighty-four bytes.

After the interleave comes the flags for ccdconfig. If you want to enable drive mirroring, you can specificy a flag here. In this configuration, I am not mirroring the ccd, so I left it as zero.

The final arguments to ccdconfig are the devices to place into the array. Putting it all together I get this command:

ccdconfig ccd0 32 0 /dev/ad1e /dev/ad2e /dev/ad3e

This configures the ccd. I can now newfs(8) the filesystem.

newfs /dev/ccd0c

Making it All Automagic

Finally, if I want to be able to mount the ccd, I need to configure it first. I write out my current configuration to /etc/ccd.conf using the following command:

ccdconfig -g > /etc/ccd.conf

When I reboot, the script /etc/rc runs ccdconfig -C if /etc/ccd.conf exists. This automatically configures the ccd so it can be mounted.

If you are booting into single user mode, before you can mount the ccd, you need to issue the following command to configure the array:

ccdconfig -C

Then, we need an entry for the ccd in /etc/fstab so it will be mounted at boot time.

/dev/ccd0c              /media       ufs     rw      2       2