When choosing a mass storage solution the most important factors to consider are speed, reliability, and cost. It is rare to have all three in balance; normally a fast, reliable mass storage device is expensive, and to cut back on cost either speed or reliability must be sacrificed.
In designing the system described below, cost was chosen as the most important factor, followed by speed, then reliability. Data transfer speed for this system is ultimately constrained by the network. And while reliability is very important, the CCD drive described below serves online data that is already fully backed up on CD-R's and can easily be replaced.
Defining your own requirements is the first step in choosing a mass storage solution. If your requirements prefer speed or reliability over cost, your solution will differ from the system described in this section.
In addition to the IDE system disk, three Western Digital 30GB, 5400 RPM IDE disks form the core of the CCD disk described below providing approximately 90GB of online storage. Ideally, each IDE disk would have its own IDE controller and cable, but to minimize cost, additional IDE controllers were not used. Instead the disks were configured with jumpers so that each IDE controller has one master, and one slave.
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
Note: If FreeBSD does not detect all the disks, ensure that you have jumpered them correctly. Most IDE drives also have a “Cable Select” jumper. This is not the jumper for the master/slave relationship. Consult the drive documentation for help in identifying the correct jumper.
The ccd(4) driver allows you to take several identical disks and concatenate them into one logical file system. In order to use ccd(4), you need a kernel with ccd(4) support built in. Add this line to your kernel configuration file, rebuild, and reinstall the kernel:
The ccd(4) support can also be loaded as a kernel loadable module.
bsdlabel -w ad1 auto bsdlabel -w ad2 auto bsdlabel -w ad3 auto
This creates a bsdlabel for ad1c, ad2c and ad3c that spans the entire disk.
The next step is to change the disk label type. You can use bsdlabel(8) to edit the disks:
bsdlabel -e ad1 bsdlabel -e ad2 bsdlabel -e ad3
This opens up the current disk label on each disk with the editor specified by the EDITOR environment variable, typically vi(1).
An unmodified disk label will look something like this:
8 partitions: # size offset fstype [fsize bsize bps/cpg] c: 60074784 0 unused 0 0 0 # (Cyl. 0 - 59597)
Add a new e partition for ccd(4) to use. This
can usually be copied from the c partition, but the
fstype must be 4.2BSD. The disk
label should now look something like this:
8 partitions: # size offset fstype [fsize bsize bps/cpg] c: 60074784 0 unused 0 0 0 # (Cyl. 0 - 59597) e: 60074784 0 4.2BSD 0 0 0 # (Cyl. 0 - 59597)
The use and meaning of each option is shown below:
Generally, you will want to mount the ccd(4) upon each reboot. To do this, you must configure it first. Write out your current configuration to /etc/ccd.conf using the following command:
ccdconfig -g > /etc/ccd.conf
During reboot, the script /etc/rc runs ccdconfig -C if /etc/ccd.conf exists. This automatically configures the ccd(4) so it can be mounted.
/dev/ccd0c /media ufs rw 2 2
The Vinum Volume Manager is a block device driver which implements virtual disk drives. It isolates disk hardware from the block device interface and maps data in ways which result in an increase in flexibility, performance and reliability compared to the traditional slice view of disk storage. vinum(4) implements the RAID-0, RAID-1 and RAID-5 models, both individually and in combination.
FreeBSD also supports a variety of hardware RAID controllers. These devices control a RAID subsystem without the need for FreeBSD specific software to manage the array.
Using an on-card BIOS, the card controls most of the disk operations itself. The following is a brief setup description using a Promise IDE RAID controller. When this card is installed and the system is started up, it displays a prompt requesting information. Follow the instructions to enter the card's setup screen. From here, you have the ability to combine all the attached drives. After doing so, the disk(s) will look like a single drive to FreeBSD. Other RAID levels can be set up accordingly.
FreeBSD allows you to hot-replace a failed disk in an array. This requires that you catch it before you reboot.
You will probably see something like the following in /var/log/messages or in the dmesg(8) output:
ad6 on monster1 suffered a hard error. ad6: READ command timeout tag=0 serv=0 - resetting ad6: trying fallback to PIO mode ata3: resetting devices .. done ad6: hard error reading fsbn 1116119 of 0-7 (ad6 bn 1116119; cn 1107 tn 4 sn 11)\\ status=59 error=40 ar0: WARNING - mirror lost
Using atacontrol(8), check for further information:
# atacontrol list ATA channel 0: Master: no device present Slave: acd0 <HL-DT-ST CD-ROM GCR-8520B/1.00> ATA/ATAPI rev 0 ATA channel 1: Master: no device present Slave: no device present ATA channel 2: Master: ad4 <MAXTOR 6L080J4/A93.0500> ATA/ATAPI rev 5 Slave: no device present ATA channel 3: Master: ad6 <MAXTOR 6L080J4/A93.0500> ATA/ATAPI rev 5 Slave: no device present # atacontrol status ar0 ar0: ATA RAID1 subdisks: ad4 ad6 status: DEGRADED
You will first need to detach the ata channel with the failed disk so you can safely remove it:
# atacontrol detach ata3
Replace the disk.
Reattach the ata channel:
# atacontrol attach ata3 Master: ad6 <MAXTOR 6L080J4/A93.0500> ATA/ATAPI rev 5 Slave: no device present
Add the new disk to the array as a spare:
# atacontrol addspare ar0 ad6
Rebuild the array:
# atacontrol rebuild ar0
It is possible to check on the progress by issuing the following command:
# dmesg | tail -10 [output removed] ad6: removed from configuration ad6: deleted from ar0 disk1 ad6: inserted into ar0 disk1 as spare # atacontrol status ar0 ar0: ATA RAID1 subdisks: ad4 ad6 status: REBUILDING 0% completed
Wait until this operation completes.